I get a lot of postal spam. Some of it is grocery ads. Some of it is Comcast or Verizon ads. In general, though, I don't want it, and it makes me feel bad about the environment, so I try to unsubscribe when I get something.
One organization that I unsubscribed from pointed me to dmachoice.org. Apparently, 80% of the postal spam in the US is from companies that are members of the DMA (Data and Marketing Association), and the DMA requires all member organizations to respect opt-outs. That means that if you go to dmachoice.org and opt out, you will likely get less postal spam.
catalogchoice.org is another organization that does similar things, but it seems like they only let you opt out of specific organizations rather than having a one click opt out from everything.
I only signed up recently, so I'm not 100% sure that it works, but it seems legit and it's worth a try!