It's recommended that you regularly audit your user list and remove any users that no longer require access to the system. One of the simplest ways of doing this is to remove users who have not logged in in the last [X] amount of time.
You can do this by visiting the Admin > User page, then sorting the user list by 'Last Login' and selecting users to delete.
Note, be careful not to delete any users who have just been added, but have yet to log in for the first time - marked as "Never logged in".
Deleting a list of users in bulk
If you need to delete a selection of users in bulk then you may consider assigning users to a new Group and filtering out all users that need to be removed:
1) Create a new Group in Admin > Groups, you may call it 'Delete'.
2) Export your users in Admin > Users.
3) In the export file, filter out users that you wish to permanently delete and replace the value in a column titled 'Groups' with the newly created Group in step 1 (ie. 'Delete').
4) In Admin > Users page click 'Import users' and import the updated export file.
5) In Admin > Users use the Groups/Admin filter to get all users that are in 'Delete' group; click delete.
6) If no longer required, you can delete the newly created group in Admin > Groups.