How do I remove old administrator?

Have just bought a second hand ibook G4 for my girlfriend. It came with existing software that I don't want to delete off the HD. I want to change the administrator so that the old one is gone but I can't work out how to change the shortname for the administrator, it is not accessible (faded grey colour). I do have the old administrator's password and this has allowed me to change most things. But it is annoying because the old administrator's name is still on the home folder etc because it is the shortname. The lock/unlock icon does not help. Please help...

Create a new Admin account, log into that one and delete the old one.

have already tried that. can't delete original administrator even once have created a new administrator. and if modify old administrator can't access and change old administrator's shortname.

APPLICATIONS > UTILITIES > NET INFO MANAGER > GROUPS > ADMINS

Select the admin you wish to remove and press delete on the keyboard. That account will now be a user account and you should be able to delete it from your Admin account.

I'm trying a lateral here because this thread is recent.
I have a family iMac G5 running 10.4.8.
At some point, a child changed their user to an ADMIN account.

Since Myspace has superseded schoolwork, I need to delete this account, but water-boarding their password out of them isn't an option, so I can't select their user to change it.

How do I delete a 2nd ADMIN account besides crashing the whole hard drive?