I hate junk mail in my mailbox. While CAN-SPAM certainly won't apply, you can refuse the mail.
Just write "refused" on it, put it back in the mailbox, and raise the flag. The mailman must take it away.
That way you don't incur the cost of disposing of it, and the USPS gets to eat the cost of having to deliver it and retrieve it... and probably to dispose of it or return it to the sender.
This is outlined in the US Postal Service Domestic Mail Manual, section 508.1.1.3, "Refusal After Delivery" and it states:
1.1.3 Refusal After Delivery
After delivery, an addressee may mark a mailpiece “Refused” and return it within a reasonable time, if the
piece or any attachment is not opened. Mail that may not be refused
and returned unopened under this provision may be returned to the
sender only if it is enclosed in a new envelope or wrapper with a
correct address and new postage. The following may not be refused and
returned postage-free after delivery:
a. Pieces sent as Registered
Mail, insured, Certified Mail, collect on delivery (COD), and return
receipt for merchandise.
b. Response mail to the addressee’s sales
promotion, solicitation, announcement, or other advertisement that was
not refused when offered to the addressee.
I've been doing this for more than 15 years. I basically pick what I consider valuable out of the mailbox, mark everything else "refused", close the box and raise the flag. The next day it's gone.
One time the mailman left me a note telling me to stop. I left him a note quoting the postal code. No more issues.
While this won't "unsubscribe" you from any lists, there is a certain satisfaction in making someone in the chain of delivery pay for the junk ending up in your mailbox.
For unsubscribing, there are some commercial associations marketers use that maintain do-not-mail lists in an effort to try and keep consumers happy. There are a few of them, but here's a couple of places to start:
https://www.directmail.com/mail_preference/
https://consumer.ftc.gov/articles/how-stop-junk-mail