Any suggestions out there on how to protect an EXE from unauthorised distribution to other sites?

Eg. a client buys a license for 3-users for their company; how can we prevent this software from being used at another site without going to the hassle of a hardware protection key?

I have heard of some 3rd party products being available; however, what works for VB & Access may not necessarily work well with VFP. So any suggestions on SW protection schemes that do work well in VFP would be much appreciated.

regards,

Alain