I’m not sure you can easily keep Wi-Fi under control in the environment you describe. If there are open, unsecured networks near yours, it would be very difficult to keep your client computers from connecting to those networks. One of the best ways to stop this would be to require all your company’s computers to use only wired access when in your office and disable wireless access entirely in your office. This may be an inconvenience, but it would ensure that your client computers would not be connected to the public, unsecured network and the company’s network at the same time.
Netstumbler and inSSIDer are a couple of good tools that could show you what wireless networks are around your office.