Can I turn off messaging between users in a workspace?

At the moment, it is not possible to turn off messaging between all users in a Workspace. It is possible to make a sort of restriction, if you add users as guests. Guests can only initiate direct messages with users that belong to the same channel(s) as they do. However, be aware that all regular members in the workspace can initiate direct messages with guests, and that way guests can still communicate with other workspace users.