You could look into creating a second Admin level user account that could be set to
be the default one on startup to appear; then go into the computer to see if you can
remove the former Admin user you created. This may require a few searches online
to see if anything like this appears; it has been done, but evidence going forward will
be sparse or sites that used to host older OS X information may lack further support.
OS X create new admin user delete old:
https://duckduckgo.com/?q=OS+X+Leopard+create+new+admin+user+delete+old&t=ffsb&i a=videos
Five Ways to Reset a Lost Admin Password (and other details)
http://tidbits.com/article/14437
The problem remains, the best and official way to remove an old OS X installation is
by use of the Disk Utility, found in the booted Install-restore OS X DVD; there are
ways to erase and also to reformat the hard disk drive; secure overwrite & other ideas.
Your current new Mac with AppleCare won't have a bearing on an old obsolete computer.
If you were trying to migrate from old unit to a newly purchased one, an Apple Store Genius
may be able to help with that. If you were to take a time machine back a decade, an Apple
Store or almost any Authorized Apple service provider could help you; some for free while
others likely may ask a fee for the tech time. The shops used to have a technical library of
all the system software (offline) and technical service & repair manuals, plus tools, etc.
Someone with a supported OS X install-restore DVD (retail version) would be able to start
the computer with the DVD in the optical drive, with C key held on initial boot, and when
the desktop window and installer interface appears, choose to use the installer's Utilities.
Those give the user options to erase, and secure erase (reformat, etc) so these tools,
the original or subsequent upgrade to later supported OS X system, should be retained
and included with the computer as the original part numbers indicate on those first DVDs.
You could check amazon or others online for retail version OS X 10.5 DVDs.
An authorized service provider who still has retail Leopard 10.5 on DVD may be able to help
resolve the issue, but most do not work on these obsolete PowerPC computers anymore...
An Apple User Group in your vicinity may be helpful, if their membership includes someone
whose interests are in keeping older Mac computers and their software alive.
You may be able to trade off the computer to someone who may want to use it, in part for
the time they may take to remove your content satisfactorily and install a new OS X in it.
If the computer is in near perfect condition, it may be worth something. Otherwise not much.
Sorry to not have a short easy method, outside the actual OS X install discs. I have some for
use with my own vintage Mac computers, and also sought to obtain spares when available.
Good luck & happy computing! 🙂