Teleporter's Splinch

You cause a fixed point in space to be protected from interplanar travel.

You can only cast this spell on a permanent portal used for teleportation (such as that created by the Teleportation Circle spell), or a location of intense magic (subject to your DM's approval). 

While the spell persists, the location you choose is warded against interplanar travel. A creature that attempts to teleport into or out of within one mile of the warded location must make a Charisma saving throw. On a failure, the target takes 5d12 force damage, and it is prevented from teleporting in the desired location. On a successful save, the target takes half as much force damage, and can teleport freely.

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the warded area increases by 1 mile per level of spell slot above 5th.