You shouldn't have to feel like the bad guy for not letting someone into the RP if they aren't good enough and their bad quality turns out to be problematic. You're the GM and you have every right to set quality standards. How do you deal with people like that? Tell them to get better about it or just don't let them in. But putting in a character limit for it is... weird. There are other reasons for a character limit which I can understand, albeit disagree with, but the logic here is just completely faulty. Below-quality roleplayers will continue to be below-quality roleplayers regardless of how many characters they have, it just doesn't make sense to deal with them that way. Just don't let people in who don't meet your quality requirements.