Hello
Best dealt with in conjunction with your local police teams. Anti-social behaviour has recently been made a higher priority by both the Home Office and the police, so they should be willing to get involved. There are various aspects of the Anti-Social Behaviour legislation they can use, from individual ASBOs through to making an area an exclusion zone and these can apply to public areas. Groups tend to have lead individuals, and are best dealt with by identifying and dealing with them first, the rest of the group often fade away.
As a start, it is likely to help the local police if you can be in a position to serve a banning letter / notice on the group should they cause a significant problem. The process would be:- they cause a nuisance (breach of the peace): you call the police; when the police arrive, explain what has happened; as they are being removed from the building, serve the notice in the presence of the police so everyone knows the group is no longer welcome. The police should be asked to record full details of each of those ejected. From that point on they can be deemed as trespassers, and any further incidents can be viewed as aggravated, and potentially used in court at an ASBO hearing. It may be that straight forward Public Order Offences have been committed. Your organisation could also then take out individual injunctions banning them from entry - breach of the injunction is a contempt of court and can be punished by imprisonment.
This is a bit of a summary, and the local Police ASB team will have more details.
It is a problem that can be dealt with effectively, but the solution is not likely to be instantaneous.