This is isn't a irrepressible problem. Looking from my eyes, would It be hard just to give warnings to people who spams useless posts mentioning that they will get ban from 1 week up to 1 month If they wont start posting full-fledged posts? Is It hard to do so? I have no idea how many spammers there are but If there are like major 100 or more spammers, It wouldn't be hard to deal with them?
Your questions can be answerd by the previous posts really. I agree a warning would be fairer and should be the first resort, but the mods serm to have tried that and in their opinion it doesn't work, so the week ban is their official warning.
Like i mentioned before I don't think that 1 week ban should be as a "warning". This should be just an action taken against spammers. Spammers should get bans from time to time, like i said from 1 week up to 1 month. But I don't think there should be permanent ban for spammers. Maybe only those who got banned several times for spamming.