The vet didn't do a culture to verify that there is no RI? As far I know, there is no way to stop sneezing because BPs shouldn't be sneezing to begin with. Assuming you have the temps and everything else good, you can try and raise the temps a few degrees overall and keep the humidity at 70-80% for a week and see if it goes away. If it doesn't, then I would suggest find a good exotic/reptile vet and get a culture done to make sure there is no RI.