Hm, I had the same problem now with several mice. I mean scroll wheel = button seems to be pretty standard this days. And I even fight with the same trouble often enough - accidentally pasting stuff into code without noticing. The only problem with disabling the middle-mouse button for me would be that it's the only way to copy-paste from consoles. Which again already doesn't work when trying to paste into the browser so I already must often do something like - copy with middle mouse button from console to some texteditor, copy it there into that other buffer - and then paste in the browser. Not to mention the regular guessing about which application will work with middle mouse button and which won't and which won't work with the other copy-paste. It's rather a mess right now in X.