Helpdesk Tó define permissions

[NPimenta]

Hi all,
I have a smf forum and i want define a permission Tó a mensage group above 100 posts, tuia users can only read.
Where i define that?
I put the topic in local permissions, and define the permissions Tó the group, but he permissions is not aply, they continue create topics, because exist a non agroup group surfe are all the members