Android - How-to unlink the main Google account from android device without a factory reset

What do you mean, by 'delete it' exactly? Your phone needs to be linked to a Google account, no matter what (as far as I know). If you do wish to change the "main" Google account your phone is linked with, a factory reset is the way. I've had to do it myself a few times.

If you just want to stop receiving Gmail notifications, go into Settings > Accounts and uncheck the "sync" option for the account you no longer wish to have Gmail sync with.

There are ways to do it without a factory reset. However, depending upon the phone, it may not work without being linked with a Google account. I'm not sure about the Hero one way or the other. See here.

Another option is that you could just create a dummy Google account, then never use it.

Easy! No Factory Reset Needed.. Go to Settings>Accounts&Sync>Then Click the google account you wanna delete(this also applies to the main account)>then press MENU soft key>then Delete Account

That's it!