If you want to permanently hide messages tab, run the following query.
delete from usm_link_launchpad_launchpad where launchpad_parent_id='my_workplace' and link_order=3;
Restart catalog after executing the query.
Should you need to revert back, run the query below and restart catalog.
insert into usm_link_launchpad_launchpad values('my_workplace','my_messages',3)
NOTE: Please take a backup of the Service Catalog database before running the delete statement just in case something is broken.