Bad tenants friends

Tenancy Deposits

My tenant has visitors who pour into her house nearly every weekend making a disturbannce and shouting in the road at 4.00am after the clubs close.
This has been going on for some time. The council are aware but they need witnesses from their own offices, which close at midnight until 6.00am.
I have issued a section 21 and written to my tenant several times and spoken to her. Police have been at the house 3 times after seriuos insidents. But they will not tell me what they are, which does not help me evict her. Once she got beaten up. Second time two people where just taken away by the police in the morning.
Can I ban her two sisters from visiting the house after 11.00pm until 6.00am? How is it done legally? Will I be acused of harrisment? Or can I just write to her and give notice I have banned them?