Someone who's rooted let us know if this works
read response #3 at
Hero by HTC:How to change or delete gmail account?
You will not be able to reconnect to Google after doing this, you will get error message "Can't establish a reliable data connection to the server" when entering sign in information, BUT
This claims if you are rooted you can fix it
Issue 3477 - android - HTC Hero - can't sign Gmail account if user skips signing when initially using the phone. - Project Hosting on Google Code
Post #16 has the answer, post #32 confirms it.
Again, not rooted, no Idea if this really works, try at your own risk.