some delay before they can post sound like a good idea, but i would like to refine this a little bit further.
Why not just create a single folder (eg. new to mangadex) where the new one could write and a write-delay for the rest of the forum?
For me this seems the better idea.
edit: another idea would be a minimum post count, after lets say a waiting period and one post (e.g. introduction in open newbie-folder) they would get write permission for the rest of the forum.