How to combat bad behavior + accusations of racism?

I work at a public library. There are two girls, sisters, who continually cause problems. They are loud, rude, don't want to obey the rules, constantly mess up the books, leave stuff scattered around the teen area, loudly tell other kids that the library sucks and rips people off, spin on the office chairs, etc. Every time we talk to them, they are argumentative and confrontational.

Normally, we simply give a warning or two, then ask a patron to leave for the day. For kids, we also call parents. For each incident of disruptive behavior, we write up an "incident report." When we accumulate enough serious incident reports, we take them to the library board and ask the board to ban the disruptive patron.

The problem here is, these girls and their parents are extremely prone to accusing us of racism. Their father did it when we charged him $3.00 for having overdue dvds. The girls have done it when we've asked them to leave after they were swearing and running barefoot in the kids section. We believe that the father is serious enough to actually sue us for discrimination, should we try to ban the girls. He certainly won't side with us over his kids on any given incident, no matter how much proof we have of their wrongdoing.

None of the staff here have EVER acted inappropriately or racist towards a patron. We're a small library with a limited budget, no security guard, no legal council and few staff members. How do we deal with this situation?!