This and other similar questions keep coming up so here's an example of how to prevent newly registered members from starting conversations and only allowing them to once they have been registered for a week.
Set the Start conversations permission for the Registered user group to No
Create a new user group - let's call it Established members
Set the Start conversations permission for the Established members user group to Yes
Create a User group promotion to promote members to the Established members user group using the User has been registered for at least X days criterion and set it to 7
The same approach can be used for other permissions and criteria.
Always remove the permission from the Registered user group and add it to a new user group, and combine it with a promotion.
Another question which comes up often is, "How can I create a news or announcement forum, in which only staff members can create threads and post replies?".
Again, it is achieved via node specific permissions.
1. Revoke the 'Post new thread' permission for the Registered user group.
2. Allow the 'Post new thread' permission for the desired user group(s) or user(s).
3. Revoke the 'Post replies' permission for the Registered user group.
4. Allow the 'Post replies' permission for the desired user group(s) or user(s).
If you wish to allow registered members to post replies to staff created threads, then omit steps 3 and 4.