The deal could just be: no phone number, but you have to pay $x/year (I guess this doesn't work with 501c3?)

Just charge $10 to create an account without a phone number and accept Bitcoin. Most people can avoid the $10 by providing a phone number, privacy-conscious people only have to pay $10, it generates revenue, and the $10 puts the spammers out of business because they don't pay $10 once, they pay $10 every time they get banned, which happens multiple times a day.

You could even automate the bans by banning anyone who gets blocked by more than two people they sent messages to, which anybody can avoid by not sending messages to people who would block them, and if it happens to someone innocent, it's still only another $10 to reactivate your account.