for tigerbeer..
i am petty sure about this....i was being cancelled before but got an NOC from my pevious sponsor right after he canclled me.then went for exit and comeback for another visa to a new sponsor....BUT in your case if you want to comeback after two years of being cancelled, automatic BAN lifted...you can go and comeback again to DOHA, no need for NOC or release...just to make sure that you waited for two years to be exact, noted the date that you were being cancelled...
BUT they can give you release only once you have a job and offer letter for a new company.(And still it depends if they will give you release from your sponsor....)
If an NOC, and they are willing to give, you must exit Doha and can comeback anytime....
BUT if none of the above, you need to wait for two years, for Ban lift before you can comeback here...