The solution I came up with was part spam filter and part fundraiser. I decided to draw more attention to the PayPal donate button by requiring new signups to type the color of the button into one of the form fields. A couple of interesting things happened.
First, the rate of donations didn't change. I've had one modest donation in two weeks.
Second, the spam ended. Overnight. It was fantastic.
Most interestingly, the little test showed fun insight into how we all see colors. When I originally wrote the test, the answer was "yellow". At the very last minute I decided that I should throw in "orange" as a possible answer as well and it was a good thing I did. Since deploying that change 15 days ago, there have been 51 signups - 26 orange and 25 yellow. I don't know why, but his fascinates me. It seems obvious to me that it is yellow. :)
What color do you see?