Teddy is right, about all you can do is try to deter them. Freeze
brands (or any brand) are great. I would also suggest keeping them out
of roadway pastures at night, or double fencing those sides.
My own theft deterents come in the form of 3, 100- 140+ Anatolian
Shepherds, which are livestock guardians from Turkey....if someone was
going to take my horses, they'll have to deal with the dogs first!
Tracy