Um... no? I think you mean "IP address banning" since I doubt PSN has the ability to do MAC address scraping from my router. If PSN does keep some sort of link table internally that corresponds with MAC addresses while shipped this is also easily evadable since... the PSN comes stock with 2 network adapters, which means 2 separate MAC addresses...
...and even IP address banning is not the answer here. Let me explain why in an anecdotal manner - okay I have a 13 year old and a 7 year old. Now lets say we have two ps3s in the house - one for each of them! Pretty sweet, huh? Well, they're all home one afternoon and all playing The Show online against different people (I have a nice internet connection, but this is a hypothetical anyway...) My 7 year old rage quits for some silly reason. Suddenly, my 13 year old is banned and cannot complete his game. Why? Because the two PS3's have the same IP address as far as PSN is concerned.
Okay, so let's take it a step further and say they develop some sort of "token" system that allows them to temporarily revoke the login status and "lock out" my 7 year old's PS3 from PSN but not my 13 year old's. What about if I wanted to play a game / buy some stuff on the 7 year old's PS3 online -- under my (different) user ID? Talk about angry customers...
So we're back to username-level bans which are easily evaded since PSN IDs are so easy to create. The answer is to play responsibly. Don't be a jerk about it. Count quits when the user leaves a game prematurely as losses and leave it at that. I'm willing to give the new sportsmanship rating a try if it allows for negatively rating users who drop off early when they're losing.