Getting a refund does not imply guilt IMO.

If this has happened in this pharmacy before to YOU... then you need to contact the proper agencies to deal with it.

Contact your State Pharmacy Board and make a complaint and detail the evidence, dates, etc that this happened to you.
They will act on it, be assured.
Be prepared to be interviewed if this becomes an action by the board...they make take a deposition from you, or ask you to testify.

I certainly would not patronize that place again.

There was a situation years ago when bottles of Zyprexa were tampered with at the wholesale level, and aspirin put inside them. That situation was because of the black market value of the drug which was over $600/60 at the time.
This is the link about that incident:
http://docs.google.com/viewer?a=v&q=...GyzDfGVU-ecQ1A

If controlled substances are being stolen at any point in the distribution to the destination pharmacy, the DEA would have to be notified and involved. If you report this to the state where you live they will probably contact the DEA as well.

A real resolution of your problem may not reside at the pharmacy at all. Substitution of a drug to a patient, is a huge offense. You need to bring the proper attention to this problem to the proper people to find out what is really going on.

Here is a more indepth description of drug tampering and how widespread it really is:
http://jnci.oxfordjournals.org/content/94/19/1424.full

While it is certainly possible that someone at WalMart is stealing controls in their pharmacy, there are other common reasons for your experience.

A full investigation by the agency who licenses that pharmacy and the pharmacists who work there, would be the best route for this problem.