ElectricAl:
"If I am aware of others doing it, I act to stop it. If, in my act to stop it, I do a crime, I have not stopped it, but added to it."

There's a legal term for this that's also well-known in the Human Resource world: collusion. From www.m-w.com online dictionary: secret agreement or cooperation especially for an illegal or deceitful purpose

If you know an offense occured or is occuring and you do nothing about it, you're on the verge of being an accomplice to it.

I say "Rat 'em out! and Run 'em off if possible!" The downside to that is that they'll set up their operation elsewhere as I believe their habit of stealing is deeply ingrained and not likely to change without a compelling reason or some EFFECTIVE rehabilitation.