[danjoc]

Bad guy is easily defeated. "I'm sorry, I've accidentally spoiled my ballot and marked the wrong candidate, may I have a new one to fill?" Bad guy's ballot goes in the trash, you fill the spare new one, you still have the original blank one to hand back to him. Bad guy wasted his time.

[corecoder]

Bad guy's friends are likely to be in there watching as the voter does what you suggest.