You could also have a gradual onboarding process, this should help a bit but might be over the top :
- first: multiple choice. You get 5 good questions and 5 good or bad answers. For each of the questions you have to answer if the answer is good or bad.
- Next: 5 known questions, that might be good or bad. You have to answer if it is good or bad.
- Finally: the moderation queue where the answer is only used as an weak signal to the actual moderators.
Success rate of 100% required for stage 1, 60% for stage 2 and just completion for stage 3.
This would leave a new user with a good understanding of what the site is for before they can spam.
It will also make their account seem somewhat valuable.
And if someone wants to pay someone to take their test, fine with me, at least the account isn't free anymore.