You could try going into the Manage Applications bit and clear the data and cache on the Google type applications. Be careful that you don't lose your contacts, you can back them up to your SD card first.
It is worth pointing out that any apps you have purchased will be associated with the account you are wanting to delete and will therefore be lost. Any freebies may have to be reinstalled.
The other thing you can do is a factory reset on the phone and when you fire it up it will take you through the start up proceedures and you can put your existing gmail account in to it at that point.
