Hello, I am solicitor with 20 years experience. I will try to help you with this.
I fear tht these kids are quite worldly wise. If they confine their activities to behaviour below crime ( no theft , no criminal damage) there is very little you can do? How old are they? Do you know who they are? If this is a local authority library (even if it isn't) then your council will have a department who deal with anti social behaviour which this definitely is. This can include sanctions against them or their parents in some circumstances.