If Someone Parked On Property I Owned Would I Get In Trouble For Closing And Locking The Gate Trapping Their Car Inside?

A friend of mine recently bought a house in town and his property is very weird in that it comes out in the middle of a line of commercial buildings. He has a townhouse but for whatever reason his property doesn't stop, it goes right up to the street behind his house and he has a little parking area blocked off with a chain link gate. On either side is line of typical in town small businesses, chinese place, hair dresser, bike shop, ect, ect.

People are always parking across his gate either blocking him in or out and if he ever leaves the gate open he ends up with people parking in his yard. If he were to start just closing the gate, trapping the cars, and going inside his house would he get in trouble? We keep joking about it but he hasn't done it yet.

Edit: Forgot to say there were no parking signs there when he bought the place, and he has added one with the the number of the garage he calls for removals when they do park there.