I understood the initial question to be about illegal letting of apartments....
If there is a registered charge against a property that will prevent a sale, but as far as I know the owner of the apartment may let it.
However, the gripe really seems to be with "undesirable" tenants, and secondly the fact that the landlord not only lets to undesirable tenants but owes service charges to the residents' association for the building/development. A residents' association has, in keeping with its by-laws, powers to sue for recovery of debt and to institute proceedings against owners in arrears. It seems this is the case in the building or development where you live.
Some developments have better residents' associations than others. "In-house" rules can be drawn up and voted upon in general meeting. These can cover such issues as tenants who cause nuisance and annoyance to others.
Patricia