I am going to go on a limb here and step out of the aparently common doctrine as stated by Panda, Barninho, Danton, Narly and a few others:
This is just a f.r.i.k.k.i.n.g. g.a.m.e. guys. We need to chill out. It is obvious Boffa is really loathing the game for the time being. I much rather prefer him to take a break now and come back later fresh than making him puke his guts out and possibly never return. This is just a frikking game. Who cares?
Plus that is why replacements are for. In a league for a g.a.m.e. where we have the luxury of having a few replacement players this should not be an issue at all. You feel it is "unfair" that he destroyed your team and his replacement is a "weaker" team that will be walked over by the rest of your Division? Tough. It is just a f.r.i.k.k.i.n.g. g.a.m.e.
Now, if this was happening at a rythm of 4 or 5 players per division on a weekly basis then I would understand your point of view. The way it is now... well, how many replacements have been needed so far? 3 or 4 in a league of 32 and we are half way through the season? Meh. Chill out guys.
I dont see why we have to "de facto" ban Boffa for future leagues as you have probably managed to achieve.
Personally? In the leagues I admin we have managed to have players accept a compromise here: My leagues do not frown upon players leaving if they so want. Real Life is already full of problems and worries to have a f.r.i.k.k.i.n.g. g.a.m.e. add to it on top by forcing you to do something you are really not enjoying at all. Whatever you want to do is fine with me. The only thing that we require from you is that you wait until we find a replacement player before you leave (unless it is an obviuous "Force Majeure" cause of course). It tends to work. Not 100% bulletproof but enough to make players leaving not a serious threat for the rest of the league. From the moment they think about leaving they know they have to consider if there is a replacement available or not and that helps. It usually gives us admins 1 week or 2 early warning to find someone.
If you couldn't see the humour in Narly and my posts, you need to re read them. Carefully.
On the other hand, as you say, it's just a game. So why take it so seriously, win or lose that you need to quit, or step away? Sometimes hilariously bad games happen, it's part of Blood Bowl, and in a lot of ways, it's part of the fun of BB. I do really think that learning to enjoy a good beatdown in this game is as important and fundamental as learning to cage the ball.
It's no different than other games which can go hilariously wrong. I've suffered utter destruction in warmachine, innumerable GW games, dominion, battlestar galactica, WEE, WiC, and squid knows what else. The point of the game is to have fun, and if you can't have fun when losing, unless you're a really good player, you won't be having fun about 50% of the time.
We've all felt like, at one point or another, Blood Bowl has become frustrating, and I hope Boffa comes back to play again, but as someone that has run numerous leagues for BB, and admined innumerable other games sessions, roleplaying etc, there is NOTHING worse than someone dropping out after they have made a commitment just because something hasn't gone their way.
I find this particularly funny, not as in HAR HAR, but as in HMM, THAT'S ODD... since Boffa had the most outrageous luck in our game 2 weeks ago. It's not as if everything has been going tits up for him.
As an additional point, when I first saw this thread title, I misread it as 'quilting' and was bitterly disappointed at the content.