A way to check if Garry's Mod is owned?

I’m sick of people getting “unbanned” from my servers because they use Steam family sharing and change their IP. I’m trying to find an efficient way to check if Garry’s Mod is really owned.

Steamcommunity’s XML feature does not seem to work. Garry’s Mod is ignored when opening the page without XML but it comes back when I enable it. And even if it worked I can’t check private profiles.
Example :
without XML
with XML

So, any ideas? <3

http://forum.facepunch.com/showthread.php?t=1341204&p=43469693&viewfull=1#post43469693
Maybe this’ll help you.

Thanks, I didn’t know about this script.