There is a flood control within the admin area which limits the number of posts non admin user can make within set time frames.
I did not mention this before as it effects all non admin users, but you could use this if you want to limit the frequency of posts for all users.
To change the suspension message text. All the text within the forum is kept in the files in the language_files folder. You can edit these in notepad to change the text used within the forum.
Also the ladder system could be used. You could place certain members in a ladder group that when they reach a certain number of posts the group they get moved to has no permission to post in some or all forums.