We all have friends and family who have added the FarmVille application at one point, played it for a while, then for whatever reason decided they didn't want to play it anymore. They may have hidden the feed posts and blocked the game requests, but we still see them on our gift lists and certain features still try to get them involved with posts that go ignored (like pregnant pigs or truffles).
How-To author Ama has written a very helpful article to send to your inactive friends so they can easily learn how to delete the application. This will help keep you from wasting gift requests, and you'll always find truffles for people who will send them back!
The best way (I think) to send your friends this article would be by sending them a kind and short message that explains why they should remove the application and gives them the link to instructions. I've written up an example message that you can send to them (individually out of respect!) if you'd like to copy paste it. Feel free to tweak it or write your own. Just remember to be very kind to promote follow through :)
"My dear friend,
I'm wondering if you can do me a quick favor? As you know, I play FarmVille on here and at one point or another you seem to have added the application as well. If you are not interested in playing anymore here are some easy instructions to help you remove the application and block all those pesky gift requests. This will prevent people from being able to post FarmVille related items on your wall or see your name on their lists for gifting.
Don't worry, your progress will be saved for up to 6 months! If you decide you want to play FarmVille again, you can simply add it back and all your items and neighbors will still be there.
Thanks so much for your help!"