The only thing I could imagine is draconian - delete the account, goto Application Management, delete the data for app - and then setup the account and sync all over again.
I know from experience that it certainly causes GMail to loose all history.
Draconian and a PITA - but after several days with no takers for ya, that's all I got.
what EM is perfect if you want to clear gmail. if you want to clear the email history from the "Mail"client you need to do this:
mail -> clear data
after you do this, it deletes the mail application data and history. you can then set up the mail account again. it will remember your email address but not your pw and history and then verify that it removed the address by composing a new email and typing the incorrect address but NOT hitting enter.
things in your email should be clear at this point.
hope this helps