If it's fairly new, I would take it back and exchange it.
Dust under the screen should not happen if the faceplate lens gasket/seal is tight on the faceplate and the faceplate itself is also snug. Press down on the outer lens to check for any movement caused by separation - there should not be any. If there is, the dust will only get worse.
If it's not returnable, try resealing the outer lens to the faceplate. Sorry, I don't know where to get the correct adhesive tape for the 8330. Maybe it's the same one as the 8300 8310 8320 listed here: https://www.gsm4world.us/blackberry-...01990_221.html