I wouldn't consider a one-time dialog box (growl, of course) to be that much of an issue. It might violate your design principle, but in the grand scheme of things, I don't think it would annoy the user.

Is Growl a part of OS X?

A default list of applications to silence on the first run wouldn't ruin your strapline. Which apps should be silenced on default, that's harder to figure out.

Somewhere upthread someone mentioned an adblock-style collaborative/hosted block list. Having it follow one (or more) RSS feeds would allow anyone to offer a list, and any user could choose whether to use it or not, without any additional effort for the dev.