yeah they can find out lol.
im guessing they had it unsecured lol, serves em right
if you download stuff (torrents or something) they'd probably figure it out i reckon. they can always go into the router settings to see who is connected etc so yeah lol go into their access point and delete logs or something, to figure it out, run "cmd" and type "ipconfig" then type the address of default gateway (something like 192.168.0.1 maybe) into internet. usually the router things have username and password combinations like admin password , admin admin, admin and no password. lol.
if they figure it out, then yeah they can ban your MAC address (sorta like a serial number), but then theres ways around that. keep in mind, that just by connecting to their network, that's illegal