I'm afraid to trigger some sort of forum's automatic response if I'll just paste a ton of XSS payloads here.
Chances are Cloudflare blocks certain posts, I haven't seen the forum itself restrict posts.
CF blocking certain posts is one and the same of the forum blocking posts. The forum is using CF as it's agent to block DDoS and other attacks. (the posts being blocked would fall under what could possibly be "other" attacks).