This is what 8chan does and it did not fill up with it. They're permitted to implement restrictions on that only humans and not bots may post, not on content,

This is what 8chan does and it did not fill up with it. They're permitted to implement restrictions on that only humans and not bots may post, not on content, provided that it be legal. 8chan had a system where one had to solve a captcha every day and where one was not permitted to post more than one message per minute.