I believe blocking IP address of spammers isn’t effective, since switching IP addresses is somewhat easy. Plus, I think this can evolve into a cat & mouse problem where developers keep coming up with new ways to block bad IPs (like blocking a range of IP addresses & more & more) and bad actors keep improving on their attacks. Thus, a relatively simple way would be to use micro transactions per post, mentioned by others.
For content moderation, the solution that moderators assigned to a pleebbit will simply delete violating content does not seems scalable. I would recommend you to check out Vitalik’s post on using prediction market to scale content curation. I am also developing a platform that uses prediction markets for content curation, and I believe should a good way to test such a system with real users. Would love to collaborate on this!
Also, I believe it’s unnecessary to have sybil resistance in social networking platform. Rather a well defined reputation signalling system, good content moderation, and use of something like micro transactions for spam protection should be a good starting point.