Provided sentences justify the crime, make them serve the full penalty, without exception. If/when (their sentence allows) them to come back, they should be able to - and live with the stigma for the rest of their lives. I'm all for punishment fitting the crime but in a civilised society, rehabilitation and the opportunity to put right your wrongs should be allowed.
I'm appalled and disgusted by what they've done to the game we love, but as Michael Johnson said re; cheating and doping in sport; if the punishments fit the crimes and are significant, you're not having the conversation about whether they come back.
Ban them all for 10 years mandatory for first-time offences - in any sport - and it's effectively career over (even for Amir, who would spend 10 years out of ANY cricket, could he come back at 28?). That is not the ultimate deterrent, but it's a very significant one. Butt and Asif's careers would be finished and Amir would find it very difficult to come back and most likely couldn't.
It also takes away appeals like this one. If the law is applied throughout sport re; fixing, doping it's there for all to see and abide by. It's the leniency of the current sanctions that is a big issue. How Olympians can test positive then be available for the next Olympics amazes me. Miss the next two and it's potentially a different story...