Players are getting too crafty with Minor Illusion

Quote from igneousguy >>

"players are using the spell (as walls, doors, images of themselves)" ?

You cannot create a minor image of a creature.  Only a stationary immovable object.

 A stationary, immovable object like... a statue of myself or another creature?

A realistic manikin is what I usually do. My illusionists usually take the Keen Mind feat. They visit all sorts of shops and memorize the appearances of many things like mannikins. They tend to practice these illusions regularly to keep from forgetting the images after a month.

I will also say remember whatever they know in the real world, they are dictated by there in game stats, I used to play with a physics professor who was amazing because he never thought above the int of his character. If they are not high int talk to them and explain they just don’t know that stuff in game. 

Quote from Mergon >>

A realistic manikin is what I usually do. My illusionists usually take the Keen Mind feat. They visit all sorts of shops and memorize the appearances of many things like mannikins. They tend to practice these illusions regularly to keep from forgetting the images after a month.

 most humans and humanoids will be slightly taller than what is allowed by the spell, yes you can say that the imaginary cube is tilted 45 degrees and thus gain more room but that is kind of working against the spirit of the spell

By you reply the chair couldn't be setting on the floor.

In my experience minor illusion isn't that much of a problem for  a few reasons

1. a silent non moving image of a humanoid is unlikely to draw much attention in combat
2. any interaction reveals the illusion  to be false. It is unclear who it is revealed to but a common interpretation is everyone aka the illusion is functionally dispelled if attacked or attacked through
3. Casting minor illusion requires an action the same as discerning it meaning that it is typically  neutral in terms of relative action economy. 
4. hiding still requires a separate action 

it's one of those things that sounds like it should work better than it actually does. In combat most enemies will functionally ignore a minor illusion of anything other than an inanimate object and any inanimate object minor illusion can at best be used for a single attack with cover.  If used to hide then it takes an action to see through the same as searching the area normally. 

I generally run illusions as follows

• illusions/ images end if attacked or if an attack would pass through the image 
• phantasms can last multiple hits but require a save
• minor illusion reliably depicts inanimate objects only. It lacks the ability to convincingly act as anything moving 
• Silent image can do inanimate objects but not phenomenon like fire or water. It can also trick people in combat but won't hold up as a living thing in conversation
• Major image can reliably trick people  in conversation  and pass as any phenomenon.
• I also let image spells work like a veil of invisibility. Things inside the area of an invisibility spell can be made invisible to those outside of it but this effect ends when some one else enters the area (as they appear to vanish).  Silent image  and minor illusion cannot hide sounds smells ect when doing this but major image can. If you have the enhanced minor illusion ability you can use this to hide sight and sound but it's still limited to a 5ft cube. The silencing effect also just an illusion and doesn't prevent spell casting or thunder damage. 

It's important that minor illusion can do less than silent image and that silent image can do less than major image. In particular silent image and minor illusion need to be significantly limited because infinite uses of both are easily obtained. This means that allot of the shenanigans people like to do most with illusions are limited to major illusion but given how impactful  something like convincing a whole town  with a speech given by an illusion of their king can be, I think it's fair that it require a third level slot. 

Here's a few tips for running illusions and general, and Minor Illusion specifically:

1. If you’re DM for an illusionist, always ask the player what they’re intending to do with an illusion, what effect they’re trying to elicit from a creature with their illusion. And vice versa if you’re a player, tell the DM.
2. Illusion plausibility: the more plausible an illusion (i.e., how well it fits in a given circumstance), the more convincing it will be to creatures targeted by the illusion, and thus the less likely the creatures targeted by the illusion will actually Study the illusion. 
3. Examples of uses for Minor Illusion:
   1. Utility: create maps, figurine/statue, portraits, or some document 
   2. Communication:
      1. create a auditory message by whispering to someone up to 30 feet away
      2. Create speech bubble (or document) with text for someone to read up to 30 feet away
   3. Concealment: providing concealment for a rogue will get them sneak attack on their first attack
      1. Urban setting:
      1. Create box for concealment 
      2. Turn a small door/passageway/window into a wall, and vice versa
      2. Outdoors - create boulder
   4. Ability checks:
      1. Create a booming sound coincide with your Intimidation 
      2. Deception: make something (not interacted with, e.g., held in your hand) to appear more valuable than it is, false documents, false evidence, etc.
   5. Aid for stealth (move silently): create church bells in background 
   6.  Combat: 
      1. For monsters, beasts: create a bloody carcass (this might work for humanoids too)
      2. For beast: sound of a bigger natural predator to scare off the current predator. 
   7. Temporary distraction:
      1. Humanoids: create a pile of gold, treasure, gems, etc.
      2. Cries of "He went this way!” from the opposite direction (ideally in the voice of some authority, e.g., a commander, king, etc. As DM I would require that the caster actually have heard this authority)
      3. Sound of swords clanging/attacking
   8. Mischief
      1. Create a wanted poster of an enemy
      2. Create false evidence
      3. Create a copy of the McGuffin owned by the BBEG and show it to the BBEG as if it's in your possession
      4. Create a chair for someone to sit on (then fall through)
      5. Cat call the princess/mayor’s wife
      6. Ghost sounds to get people out of a building