As to whether they can be forced out, it is a tricky one. Some are on public land and some are on land owned by St Pauls. If St Pauls were that bothered, they could have called the police immediately and then applied to the Court for an eviction order. Instead they have taken this rather curious step of letting them remain and closing their doors.
I support their right to protest and although I think the protest is pointless, it doesn't mean they have any less of a right. If demonstrations and protests were allowed to continue or were stopped on the grounds that someone somewhere thinks the cause is pointless, we are heading down the slippery slope to removing freedom of speech and imposing censorship.
St Pauls can take legal action and it won't cost an arm and a leg either. They have chosen to take a different route, for what reason I know not.