At the beginning of the area:

Set a variable equal to the "switch" X location
Set a variable equal to the "switch" Y location


After you push the boulder (same event):

Set a variable equal to Boulder's X tile
Set a variable equal to Boulder's Y tile
Branch if BoulderX is equal to SwitchX
____Branch if BoulderY is equal to SwitchY
_______Set switch [xxxx:AwesomeSwitch] ON
____:End
:End

Your AwesomeSwitch could open a door and render the boulder unmovable, among other things