It's very difficult in this country to evict someone with a legal tenency and any interference with electricity or water supply supply could be seen as intimidation and harassment and the law would likely be inclined to come down in their defence so, definitely do everything legally.
However, if she has no formal contract and was allowed to stay for a limited period, at your invitation, then I'd be inclined to remove her property and change the locks and let her take it from there!
The one proviso is that if she was a tenant with a valid tenancy who hit on hard times during their tenancy and was allowed voluntarily by you to live rent-free for a short period, then that would be a different case!
My daughters rent out properties they own and from experience, eventually the wayward tenant will suddenly disappear, it's the damage they cause and the costs they run up in the meantime which is the problem.
