Put some nextgen firewall between it or make sure the access points can block such traffic. I have worked with meraki AP's or sophos firewalls, they can block traffic like p2p. It will cost money....
Otherwise check the current solution, maybe it is capable of blocking different types of traffic.
Content filtering could help, blocking torrent sites, known file sharing sites, etc
Personally if you haven't already you may want to consider either ending the "Free" wifi