Build a metal cage (a Faraday cage) around your premises. A metal cage is a fairly standard way of blocking EM transmissions.
See, for example, <a href="http://howto.techworld.com/mobile-wireless/2767/can-faraday-cages-tame-wi-fi/">Can Faraday cages tame Wi-Fi?</a>
However, somehow it's hard to imagine that that's what you really want. It's rare that anyone cares if some other network is accessible from a given location. Usually the problem is reversed -- you don't want <i>your</i> network accessible from <i>other</i> locations. The solution for that is to enable and implement the network security functions provided for your Wi-Fi network. Exactly how that is done will depend on exactly what equipment and software you are using.
Or perhaps you're concerned about someone in your location connecting to some open external Wi-Fi network and later connecting back to your network. Potential malware might be imported through the "dual-connection" PC.
In that case, lock the PCs down so that only an Administrator can make networking changes on the PCs. The exact way to do that will again depend on exactly what equipment and software you are using. Most especially, each individual PC might need a slightly different procedure; many sites have a wide range of PC configurations. In addition, ensure that your AV solution is sufficient and properly updated. Other products for spy-ware, etc., might also need attention.
It might be best if you explained the <i>business problem</i> that you want to solve.