Hello ellery,
The best way to do a clean install is to format and reinstall redhat.
However, if you can't format:
delete all Users and Reseller through DirectAdmin
run: userdel -r admin (assuming admin is your login)
rm -rf /usr/local/directadmin
note that this will leave config files on your system, and some cleaning up will be required to be done, such as removing unwanted services.
We havn't included an uninstaller yet, but when time permits we'll look into adding one.
John