The question is, how. It would be impossible to remove things such as bots, spam or other bad things in a decentralized forum as there would (by concept) be no moderation. It would rely on responsibility by the posters on the forum. The internet isn't responsible.
FMS uses a web-of-trust system. (This forum's trust system was inspired by FMS.) You only see posts of people in your trust list, or in someone you trust's trust list, etc. This prevent spam, though it's tricky to figure out how to add new anonymous people to the trust network. FMS has a feature where anyone can publish a CAPTCHA, and solving this will add you to that person's trust list automatically (with a low score), but this is pretty vulnerable to DoS attacks. Paying some bitcoins is also a good idea.
I haven't looked into this stuff in several years, so I may be out-of-date on some things. Maybe things have improved somewhat by now.
When I tried it FMS worked very well for me, but again, most people don't want to manually manage trust lists.