Her dad, brothers, cousins whoever, go and drag the fucker out
Legally you can use reasonable force to remove someone who is trespassing from your property, but it's a minefield.
Dragging a trespasser who refuses to leave off your property may be considered a reasonable use of force.
However using force can become illegal very quickly. If the force is excessive, for example a mob of people dragging someone out by their ankles, someone getting dragged down stairs, or across concrete/gravel, if the use of force continues when they are outside the property, if weapons are involved or if they sustain a significant injury, then it's a criminal matter.
If the person is not trespassing (by it's legal definition) and not posing a threat, using force against them would be considered an unprovoked assault.
If the person has any legal right to be in the property, using force to remove them could also be considered an unlawful eviction.
If the little toerag is refusing to leave as some form of control/abuse, he'd likely love nothing more than to drag the family through the legal system for assault or similar, and/or compensation.
Legalities aside, as gratifying as a bit of 'mob justice' sounds, real life is rarely as simple as 'sending the boys round'.
There's always the potential for things to get out of hand and someone to get seriously hurt, or worse.
If he had any reason to be fearful of OP's family, he wouldn't have refused their request to leave in the first place. So even if they did manage to physically force him out and change the locks, what's to stop him coming back with a bruised ego and booting the door off? Or worse?