Cold Snap!

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 4th level or lower, its spell fails and has no effect. If it is casting a spell of 5th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

Moreover. the caster of the spell must make a DC CON save.  On a failure, s/he takes 2d6 cold damage and is frozen solid (condition paralysis), with a long pointy icicle hanging down from the tip of their nose.    The target takes 1d6 cold damage every turn thereafter.  They may reroll their save at the end of each turn, ending the spell on a success. The spell ends after #[level cast at] rounds, if the frozen target takes any damage, or when an ally can take an action to shake them out of their icy paralysis. 

On a success, damage is halved and the target is not frozen.  A target with immunity to cold damage automatically succeeds its saving throw; one with resistance rolls at advantage.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.  Also, for every level above 4 the spell is cast, it does an additional 1d6 damage initially and on every turn.