Ahem. I have experience in this
Make a differentiation between Mass Freekill and Spraying Stack.
I would say this, Mass Freekill is 3+ freekills in a round and this stems from being new or being careless. In my case I didn't hear warden where he told Ts to get to the marker if they want ST, I only heard the first part and once I realized the mistake I made I stopped, this also goes for new players when they join CT and haven't read the rules, and they start killing Ts without it being justified. For these cases I would say a 2 week CT ban should be enough because it should be seen as a learning moment not a punishment. If you were to make this a permanent ban they would either leave to another network or will come back on an alt because CSGO is free.
When someone comes on to spray stack this is with malicious intent and I would go with what people above have said go for a 1-3 month ban and have it contestable after a third of the sentence was paid. If they are being genuine in wanting to come back they'll either wait it out or contest it. If they want to cause chaos they'll probably go for an alt and spray stack again, and will be perma'd for ban evasion.
I'm not a big fan of an instant perma ban especially if it was a mistake.