Well, I was a bit tired when I wrote that, but where do you feel I'm "contradicting myself?"
I think Kara's hit the nail on the head. We don't really know how intense the explosion was. However, the explosion doesn't need to be nearly as potent as some of you seem to be supposing. It doesn't have to destroy the planet to trigger an extinction level event. Biological life is ridiculously fragile, really. There would be a burst of neutrinos (that probably wouldn't effect much except to really excite geeks like me). The X and Gamma-Rays would continue for several days. No clue how intense they would be since we're not talking about a naturally occuring event.
Just as a "Hey, this is kinda neat" aside, Capella is a binary star system. The two stars are about 3 and 2.5 solar masses respectively according to current measurments. I haven't checked this in many years, but I'm pretty sure that neither Capella star is massive enough to supernova naturally.
I suspect that you could arrange to shield major population centers. That's not the problem. The problem as Kara pointed out is that the burst would be damaging the atmosphere and surface of the entire planet. Also important, the burst could be a destabilizing influence on the local sun. Not that it is going to cause other suns to supernova or anything silly like that, but it might cause some pretty nasty local flares. The recent spat of solar activity we've witnessed in real life is a walk in the park compared to what a star like Sol is capable of doing if sufficiently pissed off. There's some evidense to indicate that it may have been a lot more "variable" in the past.
A sufficiently advanced civilization could probably engineer a way to protect planets from a supernova throwing their entire economy into it. It is debatable whether or not the GTVA is sufficiently advanced.