The solution is to mark their email as junk. No, they won't ever know about that. On my Mac, it means that the email goes immediately to my junk mail folder and doesn't load the attachments. If I want to, I can, whenever I check my junk email folder.
By the way, if you don't want to be marked as junk, here is what to do
• If you must send an attachment, keep it a very small jpeg. A picture of your dog won't hurt. If you don't know how to optimize jepgs, and you insist on sending them in large batches, you go to my junk folder. If you want to show your friends all of your photos from the dog show, learn how to use something like Flickr and send a link.
• Don't ever, ever, send a video. If you see something funny on YouTube, send the URL link. A link is an address. To send a link, just copy the address from the browser and paste it into the email. The address always starts with http://
It's best practice to go look into your junk mail folder every once in a while, so if you need to respond to your friend or client who sends you just with a "hey, that's funny!", you can still do that. No, they won't learn not to send, and forward, attachments, and if you try to teach them, you just might lose a friend or a client.
If you would like to get personal on-site software training in Adobe Photoshop, Illustrator or Dreamweaver in the greater Phoenix, Arizona area, please contact me. Paypal accepted, morning appointments only.