Assuming your hamster is not sneezing, suffering a nasal discharge or otherwise looking ill, in which case veterinary treatment will be required, the most likely explanation is that he is not happy in his quarters for some reason. This has caused him to rub his nose on the bars in an attempt to escape from here.

Look at the way that your pet's quarters are laid out, paying particular attention as to whether there is enough cover where your pet can sleep. Change things around, provide more hiding places and things to do, and perhaps even think about moving your hamster's quarters.

Small pets should never be positioned near a stereo or television system, because of their sensitive hearing. This may be what is upsetting him. If you notice that the scabs are more widespread though, see your vet as your hamster could be suffering from parasites.