Tao-park, see my first and recent post here called Nightmare on Executive Committee Street.
In my research (additional to Flat Chat) I found where one clever person asked for permission to read out nasty letters and emails one of the Executive Committee had written to him which suitably embarrassed the culprit and shut them up. If you or your tenants have anything unseemly in writing from your Chairwoman, you might thing about doing something similar. You could even show them to the police who could probably do nothing; but at least you could tell her you had spoken to the police. That might have some effect.