Set one as the default (options - advanced options - default services). You can't get rid of them (unless maybe deleting the service books for them), each email account gets a calendar.
Posted from my CrackBerry at wapforums.crackberry.com
I actually have a similar question. Somehow during one of my upgrades, something converted most of my calendar entries from an existing email account to Device Default. When I synced back up with my outlook, I either wiped out my entries in Outlook or doubled my entries in my phone (one event was Device Default, the other was my email account).
How can I convert the Device Default entries back to the email address?