I agree with all the other folks who feel that nothing will stop a determined thief.
I'd suggest some sort of video surveillance system, with cameras positioned to get a good view of the machines as well as the probable escape route of the thieves. Motion activated lights and an alarm that sounds inside the house would be good, too. I'm sure the homeowners assn would prohibit anything that made a noise outside the house.
Thieves suck.
While I was at Lowe's this morning I asked one of the lawn mower guys about this. He suggested to electrify it as you would an electric fence.
Oh so tempting.