My flat sell is being sabotaged

Hello,
I am trying to sell my old apartment, but some people in the same building are scaring away my customers, using rude language, gestures and threatening to "make court case" if they buy the flat. Some of these customers were quite willing to buy the flat already, so scaring them away caused me a major loss. I have tried to talk but it is impossible to get through the rudeness. These people say that they're doing it because I have not paid the due maintenance charges for the last few months. I say that for the last five years I have been paying it in six-month installments, instead of going through the hassle every month. I have never had a gap in this routine, paid honestly and regularly, and so it is ridiculous to think there will be any problem now. Also, this building maintenance committee that is behind all this, is not registered, has been operating without publishing its income and expenditures, had increased monthly demands without submitting valid documents, and has been accused of corruption for six-seven years. Anyway, nothing gives them any right to damage my sell, does it? One of my friends say this is a criminal offence. Is that right, and if so, how may I fix this without taking it to a court? Because involving a court case or police will decrease the value of the property substantially, so I would like to fix it, and be compensated for my losses, in a domestic manner. Please tell me the right way to approach this problem, minimizing friction and damage.