I had the same problem with mine, dust getting in from the top right hand corner...... You'll either have to send it back to your supplier, or you can remove the cover yourself, quite tricky, but there is a useful guide on You Tube, search for "6700c Screen Replacement"
This will show you how to take the phone apart, you'll then need to clean it out with an air duster, or lens cleaner & then apply either Vaseline or Silicone Sealer around the rubber seal surrounding the screen. It's obviously a design fault with the 6700 which Nokia should be aware of, so if it's under warranty, you'd be better to send it back.
Good luck.