What I do when someone does this to me:
I pick one of their forwards about which I have something sensible to say, and then reply to it as if it was a personal email and ask them some things about themselves and tell them some things about mine. Sometimes it works to get the "normal mail exchange" to work again.
"Hey Friend, I really wouldn't worry about Bonsai Kittens, though I agree it would be horrible if it was real, but it is not! It's a joke that has been on the internet for more than a decade now. I get plenty of those hoaxes in my inbox every day - would you please mind not forwarding them to me anymore? Speaking of pets, I have a new dog! My dear old dog died a while ago and I still miss her a lot, blah blah blah, went to the shelter and adopted this new dog, yadda yadda yadda, and how are you doing? Do you still have cats and a dog of your own? Is everything going well in your life? I'd love to hear from you soon!"
"Hey Cousin, well wouldn't that be awesome if it was real! Sadly, Bill Gates is not handing out his money to people who repost this email - I'm sorry to tell you, because yes, couldn't we all do with a bit of money? I could definitely use it to invest in my new business! I make figurines crafted out of merinowool now! I learned that for my classroom first, but now I don't teach anymore - I take it, from your facebook updates, that you are still teaching and enjoying it? What age level do you teach now? Do you have nice colleagues? etc etc, get together for a chat, blah blah blah. P.S. Would you mind not sending me forwards anymore, please? I get so many of them in my inbox they drown out my real mails! Thanks so much in advance! "