How to disable Alt-Left Click windowing?

So, I set up my Magic mouse to take advantage of the "highlight-paste" feature of X11 by having a gesture (click 3 fingers on the mouse) on the mouse do the Alt-Left Click to paste what I've just highlighted. This gesture is easily done on accident, and now I've discovered this annoying windowing feature. If I move to a new window and accidentally click with 3 fingers, it causes the previous program I was in to disappear. I can quickly return to it with a Command-tab, since it did not quit the progrm, but it is annoying and I want to turn it off.

I've looked in the keyboard shortcuts in the system preferences - but those deal with keyboard-only shortcuts, not keyboard+mouse actions. I can't find it in the Mouse section of the Sys Pref, either.