You host a forum, a thread or a box chat and you try to drive the story as the way you want and there is no way to do that? Well, please don't worry because we would like to introduce for you this add-on "Limit Inbox Messages".
By setting a limitation on the number of messages in the box chat, from that time, people could not reach out of the limitation. If the users reach the limit set by admin then they will not be able to send and/or receive new personal messages. By doing this, the users must concentrate on the main title of any talk.
However, they still can reply to the messages that are in their inbox. To be able to send and/or receive new personal messages they will have to prune some of the older messages in their inboxes. Admins and moderators will still be able to contact users even if they have reached the maximum inbox quota.
When someone tries to send them a personal message and won't be able to do so because of their inboxes being full, they will be notified of that fact via an alert.