ggg what a nightmare
They sound like anti social little twats who deserve a good kicking IMO.
However as that is not very civilised I think that speaking to the police is a good start. Do they have the equiv. of community police in france, who might hang out in the square a bit so that the troublemakers didn't feel so comfortable there?
Do you have neighbourhood watch or equiv? Maybe it woudl be a good time to start one if not.