You need some video surveillance! If they are that determined, they will probably just break off the Lanloc tab.
Not sure if you can do this with the gear you have, but I've done this in the past. If you can, rewire the jacks in the conference room so the lines into it are reversed. This means a normal Ethernet cable won't work. You'll need to get a "patch cable". A patch cable will work with a reversed jack and any PC or laptop because it's wired differently. BUT, if they steal the patch cable, it won't work anywhere besides that conference room.
You could also try buying cables that are some obnoxious bright neon color. That way you can spot them easily as you walk around the office.