Ignoring an OSS option is not the crux of the issue, it only adds more stupid to the cake. They don't deserve a place on the OSS hall of shame, but on the list of shysters and fraudsters.

A company is exploiting your free-trial offer, defrauding your project of resources even if it is only a buck and a half. Why are you sending them money? Just shut them down. Unless you have some really unfortunate wording in your TOS, there is nothing they can do. On Monday, send an e-mail to all accounts associated with $COMPANY and tell them in clear terms that you are going to terminate their free-trials COB EOW. Leave a special contact number to negotiate fees, wait for your phone to ring.

Seriously, why are you putting up with this?

Yes. Your pain point is the level of support you are giving on good faith to these charlatans. Maybe add a clause to your license that you reserve the right to limit the amount of technical assistance on the free download. Outside of that, if I understand this matter correctly, most of your customers are honest and you’re willing to write off this one company in the interest of keeping your own corporate sanity.

To be fair to them - they've been doing it for 10 years without a problem!