Call him out on it and see if you can find any proof before firing

Tell him to kick rocks.

It'd be great if you caught him on video and could show him the footage.

sadly, if he won’t stop even after a confrontation, he should be fired.

Since you mention you have proof, then nothing left to do but fire him.

I would lay him off. Most states are "at will" employment and laying off doesn't require a reason. Just cut ties and move on.