Cant Twitter or any other social site just have algorithms that block spam/bots? Like Gmail. I get hardly any spam in my inbox because it's smart enough to categorize junk email right off the bat. If any sneak through, they eventually label them junk too if enough people block or report spam.
I get the gist social sites dont really care though because they need bots because it adds to the user count.
But IMO a fee would do wonders. It's like dating sites. Sign up for free ones and you get endless junkie users and people spamming sites. Do a paid one where you cant really do anything unless you sign up for a membership of $20/month and you realize all the people on the site are more professional looking, look like they seriously are looking for a partner and you'll see no junk from people posting to escort sites, hoping they check out their instagram account, etc.... You get users who truly want to use the service in a serious and mature way.