unwanted 'visitor'

Tenancy Types and Management

we rent out rooms in a student house on room only basis.
after having tenants who didnt use the heating in winter which resulted in the house getting damp we decided to increase the rent slightly and pay the gas bill ourselves.
however it seems that one of our current students has his girlfriend staying all the time. we have had to visit recently to attend to repairs cleaning etc and she is always there - using the shower etc. when challenged the student says she is only visiting. we suspect she may actually be living there. we are not sure how to manage the situation because apart from increase in gas costs and metered water costs there is the extra wear and tear on the house. the house is registered as an HMO and we are limited to how many tenants we can have. i did think we could ask her to prove that she has another address. if she does we could ask the student to make a contribution to the utility bills.
i realise she has no legal right to live there but not sure how we can stop her.