There are a few ways to do it on IE/PIE.
As Chiark mentioned, one way is to trap/hook the key presses and filter out ALT key presses meant for PIE. That's by writing a system hook.
Another way, done by some web sites, is to have the web site disable it by trapping on right mouse click. This will *not* work for the older PIEs, as they do not support javascript. So ok, I guess that brings us back to the first option of system hooks!