The police are correct. If these trucks are in violation of HOA bylaws, the homeowner's association should consult with an attorney about how to legally enforce their by-laws.
An attorney-client relationship is NOT created by the answering of this question. If you would like to discuss your matter more fully with my office, please contact me to set up an initial consultation.
The Police have no authority in enforcing homeowner's by-laws. These By-laws are of a civil nature and not that of being criminal. The remedy is to bring suit in civil court for these violations where the penalties are of a monetary nature and not that of a crime.
The police can't help. Send the offenders certified letter with a deadline to comply or the HOA will begin towing them away.