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).
It's often unintentional, I think it should be possible to post javascript within code-tags for instance. If you stumble upon such problems, PM theymos:
Fixed, thanks. Whenever this happens, please email or obfuscate-then-PM me what you posted. Cloudflare has a ton of rules, some of which seem useful (like detecting fake robots), but others do stupid stuff like that.