[ChillerX]

I have a plugin which can set their dmg to negative but its manual, very old and can`t remember if it would kill them. Will test and share if its working that way.

Automatic detection such as the one you are suggesting will never be accurate and you will end up messing up legit clients. Even worse you will aggravate a real cheater with any type of similar counter measures and there will be absolutely no stopping him afterwards.