Go into your account via cingular.com and then go into your email setup page. Click and edit the email address that you want to change and erase the preloaded signature that says, "sent via Blackberry". You can even change it if you would like. After you erase it make sure that you save the your settings before you exit that screen. And don't forget if you have more then one email address on your blackberry you will have to do the same thing for each one. Hope that helps you.
Or you can go to your email options on your bb and click on email settings to change it there.
This is not accurate.
You have to change this setting from your carrier's BIS site. BES users can change the autosignature on their devices in email options, but no such option is present for BIS.
Actually, last time I checked I could not even change the signature when accessing the BIS site from BB Browser. It was grayed out and had to be done from a desktop.
Posted from my CrackBerry at wapforums.crackberry.com