mouse-wheel-click-event

mouse-wheel-click-event is a variable defined in `mwheel.el'.
Its value is

mouse-2

Documentation:
Event that should be temporarily inhibited after mouse scrolling.
The mouse wheel is typically on the mouse-2 button, so it may easily
happen that text is accidentally yanked into the buffer when
scrolling with the mouse wheel. To prevent that, this variable can be
set to the event sent when clicking on the mouse wheel button.

You can customize this variable.