I want to open a level without activating BeginPlay event.
simple answer you cant, at least ive never seen a way to and it would make no sense to. however depending on what your aiming to accomplish there may be another route to get to your goal so to speak. so whats your intent; what are you trying to accomplish?
you could always just use a variable in the game instance to control the flow of the begin play script. this wont disable begin play but it will make it not accomplish anything.