When you report spam on a legitimate newsletter or mailing list, they”ll help you unsubscribe. After clicking report spam, you’ll see a little dialog like this

From the official blog of gmail said that

This only works for some senders right now. We’re actively encouraging senders to support auto-unsubscribe — we think 100% should. We won’t provide the unsubscribe option on messages from spammers: we can’t trust that they’ll actually unsubscribe you, and they might even send you more spam. So you’ll only see the unsubscribe option for senders that we’re pretty sure are not spammers and will actually honor your unsubscribe request. We’re being pretty conservative about which senders to trust in the beginning; over time, we hope to offer the ability to unsubscribe from more email.

I try to do that. But unfortunately, I don’t get that a little unsubscribing dialog. But, this is one step forward for gmail service.