garbage's unplugged set at y100- http://garbage.shattereddreams.net/thevault.html
blondie, the go-go's, souixsie and the banshees, the pretenders, and even the B-52's were lumped in as punk at one point or another. You're trying to define punk as a particular sound as opposed to a rock philosophy and approach. You don't have to sound like the Circle Jerks or Black Flag or Minor Threat to be a punk band.
'punk' is all about the attitude of the circa 70s/80s movement.
see: iggy pop
if not involved in said movement, classification could be seen as 'pop-punk' or 'punk revival'