You know the charity, you know the person. If they are using the account at all in the business of their job at the charity, simply call their immediate supervisor and ask them to please stop abusing your service.

You could also twist the knife a bit and suggest that maybe they could have a free account donated if they just stop abusing the trial accounts, submit a polite written request and then provide a receipt. (Which you can write off as a charitable donation.) Situation resolved, scumbag humiliated, good deed done, tax write off included.