As far as my understanding goes (and I've spent a day discussing it with IPFS authors in person this September), IPFS doesn't store or forward anything you haven't explicitly requested. So you participate only in sharing things you're aware of (and bad things are rather easy to remove from that cache) — and that's a deliberate design decision.

Things in local IPFS cache can indeed be "garbage-collected" (and there's a CLI command to trigger GC manually) — but IPFS daemon has a concept of _pinning_, and pinned IPFS nodes won't be collected, and will remain stored (and being shared) as as the pinning goes.