In order to deactivate users, you'll need appropriate permissions.
Most users with Admin or Super Admin permissions are able to deactivate users.
Once you've confirmed your permissions you can deactivate users under Settings -> Users
Double click on the user's row to bring up the Edit User menu, under their status you'll want to choose the dropdown option "Inactive" unless you still need to see them in filters (in which case you want Inactive Visible).
Afterwards you'll want to click "Update User" to commit the changes. They'll vanish from the active user list. If you need to reactivate that user later on you'll need to filter for inactive users.
You can now use this seat for another user for the rest of the month or contact us before month's end to reduce your seats for next month.