Using the 80-20 principle. 20% of your employees will cause 80% of your problems.
I was in the Loss Prevention arena for over 25 years.
To answer your original question "How do you guys prevent employees from stealing inventory?" Answer - You can try and deter with everything you can - video - fake cameras - reward system - etc. There will always be that one person that will simply take what is there because it is in their nature.
If you ever do catch someone, the best advise is to prosecute to the fullest.
Let your employees know that you will prosecute any illegal activities.
You also have to look at the $$$ involved.
Any loss is lost profit - but at some point you have to stop being MR Nice Guy. You bust your butt to make a 4% profit margin. If your loss through theft is .05% of your total operating cost, What percent of your profit has be lost.
Now you expand your business and you are handling more expensive and desirable product, now what?