Copy protection for enterprise software

[spiggytopes]

I'm expecting to make a first sale to a large financial institution shortly, price in the mid-five figures. It's highly specialised risk management software, but is the kind of thing a user might copy onto a memory stick and take to a new employer or a different office.

Some sort of copy protection, so that it can only be used on one machine at a time, seems like a good idea. What's the best option - a dongle, software...? (I know that no option is perfect, this is just to prevent casual usage elsewhere).

Thanks!