You cannot delete the account that you are logged in on.
Try creating a new account with admin rights, log in with this new account, and you should be able to delete the older accounts.
It would be much more secure to wipe - partition - reformat and reinstall the OS from the install disks.
This leaves the new owner a clean slate, and the only way you will not leave a trail of bread-crumbs behind on the hard drive.
I succeeded in duplicating your situation: I created an user, and logged in using the account. Then I switched to my account, and tried to delete the new account. The minus sign was gray. Then I switched to the new account, and logged out. Then I switched back to my own account and could remove the new account.