Get a black rubber bungee cord. The good strong kind.
Stretch it between two eyebolts, set into opposite walls. Make sure to set the eyebolts into studs, not just plaster.
Stretch the bungee back as far as it will go. Secure it to a stack of cinderblocks using a cord, tied in a slipknot.
Tie a brand new sock to the end of the cord, just a few inches above floor level, making sure that tugging on it will release the bungee.
When the offending gnome tries to take the sock, the bungee releases, snapping the gnome across the room, and into a waiting net.
The only problem is that if you let the gnome loose after that, he’ll tell all his friends, and they’ll all want to try it.
But then you can start charging admission, which can more than pay for your socks.