Penny,
When I lived in a co-op in NYC we had a similar problem which was solved by going to court. It was long and drawn out but the co-op won and when the "losers" asked for the co-op to drop our claim for our legal expenses, we refused. Sometimes, a sharp slap on the wrist and the wallet is all that is called for.
They had no grounds for violating co-op rules - reasonable rules that 100 other familes kept. It was very much an "I am special" syndrome.
Mary