The
House Rules thread has some ideas for revising the Thief. I don't endorse any of them and in general we usually discuss the rules as written outside of that thread, but it's certainly worth looking at to see how other groups have solved your problem.
If you keep the Thief's abilities alone, however, you still have some options.
First, make sure you're actually playing by the rules as written. The Thief has to discard a card whether or not the theft is successful. Over time, this should dissuade the Thief from trying too often, because (as in many card games) you can't win the game if you don't have a lot of cards.
One strategy you've already discovered is to gang up on the Thief (which, if the Thief is acting the way you describe, is a case of reaping what you sow). If the Thief is being a jerk, let him face those big monsters alone, even buffing them if need be.
Another option is to try negotiations: "Hey, if you leave my stuff alone, I'll help you for free the next time you need it," or "I've got a card that will wreck your game - leave me alone and it'll never get played on you." (Extra credit if you have no such card and can successfully bluff the Thief's player.)
You could pull the Thief cards out of your deck. That's a bit drastic but it certainly solves the problem.
Ultimately, your recourse is to refuse to play with someone who consistently makes the game less fun for other players. That's true no matter what the game is and what they're doing to screw up your good time.