The only way I can think would be to erase and deregister the device under, Settings, Device Information. You will lose all saved data on your apps and have to reconfigure everything though.
It might just be that some app like a RSS Reader is downloading data and filling up space.
You can click on the apps to see further details.
Go to Settings and check.
There's also an App called App Killer in the Nook Store that you can get to see what apps you have running.