Hello
You can only remove if you have admin right.
Try this:
Type sysdm.cpl in the search on the taskbar and hit enter, go to Advanced tab, click on the "Settings" button at the User Profile section. Then delete the old user profile. By doing this, you remove the user folder that is listed C:\users\old username (assuming there's one there).
Then type netplwiz in the search and press enter. When it is launched, delete the old user name.
If the old user name is "Administrator", then you cannot just delete it but you need to disable it instead. In that case, Please type cmd in the search on the taskbar. Don't press enter key yet. Instead, choose "run as administrator" to launch Command Prompt.
Then run this command to disable it
net user administrator /active:no