While BlackBerry devices are great for receiving email from lots of different addresses while you're on the go, the process of getting rid of an address when you no longer want to tote a particular BlackBerry around along isn't so obvious.
In order to uninstall an address from your BlackBerry Pearl 8120, for example, you'll have to visit the BlackBerry Internet Service link provided by your carrier (such as https://bis.na.blackberry.com/html?brand=mycingular, which is the link we used with an AT&T device).
You can use that site to remove the email addresses you've installed. Simply log in, and from the list of active address, use the "delete" button on the far-right to remove the ones you no longer want. If someone else is going to assume use of your device, it's probably best to leave the blackberry.net address installed.
If it turns out you actually need a different address to begin downloading to your device, this is the location to visit. Click Set Up Account and follow the steps for entering your account information.