what programming language are you using for the forum? Are you able to program changes?
Set up a counter to track number of posts, and posts that are not spam by username, Also set up a boolean variable for the username, whether to authenticate posts by email or not.
Until a new user successfully posts acceptably, require them to authenticate posts through email before it shows up in the forum.
User submits post,
display a message, that since they are new, they must authenticate who they are with an email reply auto generated to their email box before the post displays in the forum.
once they get the email, they submit the link that it is really them
then the forum post lands in forum, color coded for moderators to approve.
If a moderator reads the post and believes they are legitimate, they can choose to allow the
forum post to be viewed by everyone. They can then allow the user to post without authenticating in the future, or if not sure, keep them on the email rule until they appear legitimate.
If they fail to authenticate, throw out the forum post after 1 hour automatically so it doesn't even land on the forum for a moderator to see. If the user never submits a successful post, after 2 weeks, auto delete their username.