This will allow you to enable/disable the messaging feature at a sitewide level. Please note, this will affect all users in the system.
Step 1: Log in as an administrator and select Administration
Step 2. Select Site Administration
Step 3. Select Site Administration tab
Step 4. Select Advanced Features
Step 5. Scroll down the page until you find Enable messaging system. The messaging feature is enabled, by default.
Step 6. Tick/Un-tick the checkbox, to enable or disable the messaging feature.
Step 7. Scroll down to the bottom of the page and select Save Changes
The messaging feature has now been enabled/disabled.
Enabled view on user header panel
Disabled view on user header panel