1. Log into the Sage 50 company as sysadmin, in single-user mode.
2. Go to Setup, Set Up Users & Roles.
3. Enter the sysadmin password.
4. Select the user you want to delete.
5. Select Remove User.
6. Choose Yes when asked "Are you sure you want to remove this user?"

See How to remove the password for the "sysadmin" username? to remove the sysadmin user as well.