Bingo. No easy answers here... but for starters:
1/ you need to be 18 to use one of these platforms. Drivers licience/passport required in the sign up process, not too dissimilar to anyone of us signing up to a bookie account these days.
2/ no hiding behind a pseudonim... posts need to ’belong’ to someone. Accountability and visibility. Not so difficult to administer accurately if point 1 is implemented.