It's pretty effective actually, this is why they do ban waves.

One of my friends in high school used to cheat on a popular video game. The fact ban waves would occur about once a week to once a month meant whenever his accounts got banned, he never knew why exactly and wasn't able to stop it the next time.

Of course, if ban waves are too long apart then yeah you're just letting a known cheater wreak havoc on the playerbase.