One small word of caution - just as everyone one on REINSpace says you should speak to your accountant to confirm the advice you hear about tax issues, etc., the same holds true for legal issues. This is really an employment issue and you would do well to speak to an employment lawyer about what it is you intend to do, and the proper way to do it.
My understanding of the law is that unless you have a document signed by the person saying that they are responsible for any lost money AND that you are entitled to deduct same from their wages, you are NOT legally entitled to withhold anything from the money you owe them. This may vary by jurisdiction, but that`s how it was explained to me about the Alberta Employment Standards Code. The examples used were wait-staff in a restaurant who "lose" some of their float or an employee who is being let go but still owes the company for a loan they received. Without the express written consent to withhold, you are obligated to provide them with their pay, and then take up the matter of money owing to you in another manner, such as small claims court.
Also - in my humble opinion - the "how" of what you do is every bit as important as the "what" you do. Your chances of getting dragged into some kind of BS legal issue increase proportionately with the negativity of your attitude towards the person. You are a business person, behave in a professional manner.
Even if you believe the person is an outright thief, what extra benefit do you get (besides maybe a little personal satisfaction) by treating this as anything but a business issue? Treat it as a performance issue, determine if the performance issue is worthy of termination based on a careful and reasoned analysis of the employee`s history with you and your own evaluation of the facts of the situation at hand (such as the aforementioned suggestion of finding out if there is a police report), and then act on your evaluation in a PROFESSIONAL manner.
(Again, my suggestion is to run it past legal counsel before you act, but if the history and facts support dismissal and you don`t intend to withhold anything from final pay, that may not be necessary).
Remember - even if you are completely in the right, you can still be sued and have to spend money defending, even if they won`t be successful in suing you.
My $0.02 as someone who deals with employment issues a lot...