Open a command prompt (cmd)
Type in
Net user administrator /active :no
If that admin account appeared out of absolutely nowhere on your system recently and this doesn't solve your problem
Open control panel
User accounts and family safety
User accounts
Click the account you want to remove
Delete it, remove it, or disable it (only do this if you are 100% sure none of your files are on it)
You also should never delete the admin folder if it is the only administrator account on your system, every system needs 1 admin account on it