Dual-license. Set out a commercial option. Once compliance is achieved they are liable for the commercial licensing fees for the period up until that time. It i

Dual-license. Set out a commercial option. Once compliance is achieved they are liable for the commercial licensing fees for the period up until that time. It isn't a fine, it isn't damages, it is just the normal cost of doing business. May the standard licensing costs quite high (but not exorbitant) and state that other terms are available if negotiated before using the software. If they refuse to pay and the amount is large enough, sell the dept to a collections agency and at least cause some inconvenience (though that last part probably won't work across international borders). Of course if you don't want financial recompense, and open-source-ness is your only concern, then compliance it the only matter to be concerned about. But some people and commercial entities simply won't respect that up-front (so you'll have to fight for compliance) unless non-compliance has an associated cost.