I have removed the messages of my 8700f, i might have missed a quick and easy way, but this worked.
i backed up my bb using the backup and restore tab on the desktop manager.
When this was done i clicked on the advanced button which brings up a screen split into two, one side is called desktop file databases, the other is called device databases, i deleted the messages out of the desktop databases side, and saved the changes to the back up file.
On my 8700 i opened options/security options/general settings, opened the menu and selected wipe the device, when it had finnished my bb rebooted.
I then restored my data via the dektop manager, it displayed a message asking if i wanted to overwrite the databases on my device with the back up versions, clicked yes.
My bb is back exactly how it was and the messages have gone.
I lost no third party apps, the only thing i lost were the RSS feeds on pocketday.
Long winded i think, but i'm new to using the bb and it worked for me.
If anyone knows any better/quicker way can they post it.