I revived an old iMac g5 but the old owners login is still there?

See if you can create a new administrator account by restarting the Setup Assistant:

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Be very careful to notice the spaces in those Terminal Commands.

Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

I understand & remember quite well the 1st time I did it, but there's not another way as far as I know. :)

Paste these lines into Text Edit so you can arrow thru & see the spaces...

/sbin/fsck -fy

/sbin/mount -uw /

rm /var/db/.AppleSetupDone

If the PW is for an encrypted Drive then we'd need another ork around.

Fantastic, great news my friend, take care! :)