Yeah once the google account info is in there, it's stuck unless you completely reinstall. It's fine if it's your phone but once it is sold then it's a bit of a pain.
Do you know what ROM it is running? I walked someone through reinstalling in this thread, but that was using the prj Clean Desire ROM (which is a sense based ROM):
Unrooting the HTC HD2
I'm not sure if you are running Cyanogen, or something else, or even how tied you are to your ROM. But if you want to install the prj ROM then you can follow my instructions in this thread. Anything else and the installation steps might vary a bit, but flashing a new ROM is surprisingly easy.
Post back if you have any specific questions about flashing.