Have you thought about leaving them a note?
Just something like this on a big poster board that you can fasten to the fence:
"Hi, boys!
We know you've been hanging around the back fence looking at my sweet dogs, but you always leave before anyone can say hello. If it's because you love dogs and are curious, come on up to the front door, and let's talk! We're always glad to meet new dog lovers.
BUT:
If you have bad intentions toward my dogs, you should know that a friend of mine practices voodoo, and anyone who hurts my darlings in any way will have something very, very bad happen to their family jewels. THIS IS NOT ANY KIND OF A JOKE. My dogs ARE protected by a spell. Leave them alone for your own health and safety."
Something like that...
I'm stalking the St. Bernard that moved in across the street right now, but his owners are never in the yard where I can like, approach them and say, "Oh, I love your dog", and strike up a conversation. I wish they'd notice and leave ME a note on their fence...