When i checked the Space Blaster Example game, there was a code there preventing it.
At least thats what the code comment said.
Just make a Event called "Keyboard: On Space Pressed" and have nothing in the Action section.
This is what the game code comment said, quoted.
"This event is intentionally empty, it simply stops the browser scrolling down when you press spacebar (Construct 2 prevents browser shortcuts if any key press event exists for it)."