The best solution would be to vent it through the roof. A studor vent will work but by code it must be accessible. Even if you get the vent into the attic space and put a studor on it there that would be a much better situation then you have at the present time. Can you tell me how this happened in the first place?
Thanks much for the help on this. One point of clarification, it's only necessary to simply turn the stem. No need to try to turn the whole cartridge like I started to do.