Terri, I'm so sorry honey, that really sucks. We have a problem around here with people stealing garden gnomes and Canadian flags (real fuckin' patriotic to *steal* a flag.) Short of putting barbed-wire up, you can put some oil/lubricant on the fence to deter them. Because once they start to climb, it will be slippery and someone will be more likely to notice something going on.

My dad did that with the chain-link fence we have out back... put some really thick lubricant on it so that when the teenagers started to climb it, they slipped. Of course it was dark out and we heard one of them exclaim, "Eww... it's covered in dog shit." LOL!