If you’re not going to use a user account on your computer again, but can’t delete it for some reason, then emptying it is the next best thing to do.
Note: Save anything you want to keep before you start deleting things. These are destructive commands which delete all of the files and settings in the current user’s profile. If you are at all unsure, consider using a file browser to clear out the profile instead.
cd %USERPROFILE% del /A / F /Q /S .
Linux or Mac:
cd ~ rm -Rf .
This will make sure that the disused account no-longer wastes any disk space.