I've had this issue with a cousin and a casual friend about 10 years ago. I used a phased approach:
"Dear Cousin/Friend, Thanks for taking time out of your day to think about me and forward me this
offensive, inane, thoughtless thing. The truth is, my computer has been crippled by viruses and I no longer open any email that has RE: or FW: in the title, unless I know with certainty that it is in relation to a conversation I've already participated in.
I care about you and am happy to keep getting updates about your life and your family, but I won't open up any of the other stuff."
"Please take me off your mailing list. I'd rather not have to block your address."
I never received another email from Cousin and I had to block the Casual Friend. Years later, I'm friends with both on Facebook, though Cousin is hidden from my feed. Casual Friend is actually a very polite and pleasant Facebook user...
The Dude (my DH), got the Phase 1 warning early in our courtship and was pretty good until we moved in together. After two months, he got an earful at dinner time when he tried to convince me that my refusal to read offensive (to me) materials that he wanted to send me, made me closed minded.
Now, I would just delete without opening and never mention it again. I believe it's my responsibility to manage my inbox and Facebook feed and as much as people have the right to speak their mind, I have the right to put my fingers in my ears and sing "LALALALA."