Make sure they are not in then force the door, chuck all their stuff in the front garden and make the house secure and start doing what you need to do. Wait for them to return and then see what they do, if they start trying to break in call the police to report a burglary in progress. In the ensuing argument just claim that the stuff was in the garden when you arrived and you fixed the locks because they were broken. The squatter needs to prove that he has been living there to claim squatters rights but it's unlikely they have been paying a utility bill so job done. You might want to get a burly friend to stay there a few nights just in case they attempt to return. Other alternatives are, as Ted Max says, beat seven shades out of them or go through a lengthy court battle