We don't remove users otherwise all of their history in the system would vanish too. We simply deactivate users, which is done for users who have left the company, left the project, taken vacation or are having sick time. Simply switch the value from active to inactive...
...and the table.
When complete, you will notice that inactive users will be automatically timestamped with their inactivity date and time.
If the user becomes active again on the project, simply revert their status back to active and click .