Go On ... I Dare You
It's not your fault they're there. You have no way of knowing they want out — unless they tell you, which most never will. So ask them. Periodically, give your donors some options about the relationship, including "Please remove me from your mailing list."
You won't get many takers, but those who leave will overwhelmingly be people who don't give anyway. Here's the even cooler part: When you give donors choices, they become better donors — even those who don't exercise any choice at all!