To restrict PMs based on post count on the forum, visit the admin panel and enter the Settings section. Select Forum Settings from the following menu. While in the Forum tab, find the field named Restricted PM Post Count.

The number entered here determines the minimum number of posts a member is required to have before they can send personal messages to other users. Once the user's post count is greater than or equal to the value you've set, they will have access to the personal messaging system.

Once you've entered a value, this setting will save automatically after a few seconds pass.