To take a larger perspective on the issue:

Our Military Academies have an honor code that reads:
"I will not lie, cheat, or steal; nor will I tolerate those who do."

Now, look at that last part: I will not tolerate those who do." This is exactly opposite the ethic in some quarters of "minding your own business."

The first step in stopping this sort of thing is to not tolerate a thief. A man shows up with a toolbox of differently marked tools, you have to wonder. The guy just happens to have several tools that are usually company provided, you have to wonder. Some "supplier" shows up, with amazing deals in the trunk of his car, you have to wonder. Anazing deals at the flea market, pawn shop, or E-Bay, you have to wonder.

I have to doubt that these thefts are the activities of hard-core criminals, randomly driving around, who just happen to have the tools with them, and just happen to drive by your site right after you've installed the wire.
I'd look for an "inside job." Someone near you is the thief.

And that's the real damage a thief does: he makes every man distrust his neighbor.

Otherwise, look to your jobsite practices. One site I was at had continual small losses ... up to the day a neighbor 'inspecting' the site was challenged. His visits stopped - as did the after-hours losses. Coincidence?

Likewise, open your ears. One site had some guys set up a poker game at lunch. One guy had a knack for losing. Things started disappearing. The thefts stopped the instant he moved on to another project. Coincidence?
(Gambling's morality aside ... this is Nevada ... where there is no shortage of LEGAL games to join!)

The same applies to other after-hours activities. I'm sorry, but no good can come from a man who hangs out at bars, does drugs, or who always seems to have all sort of 'special deals' after work.

And, employer, look to yourself. I've known guys who prided themselves in their ability to bully / cheat / manipulate their crews. Crews are not stupid, and WILL collect ... one way or another. Many a thief has started out "just collecting what's due him."