How-to unlink the main Google account from android device without a factory reset While setting up my HTC Hero phone, I have entered my gmail account in the setup. But now I want to remove it (I don;t want to change with another account). How do I do it ?

I have searched a lot on Google and everywhere I found that I have to do factory reset but this is not my favoured option.

If there is another way, please let me know.
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.