It is generally not necessary or possible to entirely remove users from your organisation.
You can either:
Remove the user from any groups
The user will be in the In no groups tab. They will still be counted in some reports and dashboard statistics but this may be good for keeping a historical record of your users and course completions. For example, you might want to know how many users completed a particular course over all time. If you keep users as In no group you still have access to this information.
Removing the user from a group does not remove their courses or licence. However, when their year's licence expires, the courses will be removed automatically.
Archive the user
Archived users are removed from all progress and completion statistics, including Excel reports. It removes all the user's courses. This is the closest thing to removing a user entirely. You may not want to do this if you want to keep a record of course completions over all time. So consider whether you want to archive your staff members or simply remove them from any groups.
To archive a user, in Edit mode, select the user and go to Status. Click Archive. Archived users can be unarchived at any time in the same way.