If you block the spot with your body, its fine as long as you're standing there before anyone jumps towards you. If you move forward to block when someone attempts a jump, that is an obvious attempt on their life. That would be KOSable. If you push someone in a way that could lead to their death or getting damaged, that is also KOSable.
I'm bumping this Q&A to clarify a point. It is NOT traitorous to simply move into a blocking position unless there is no other place for the jumper to land. It only becomes traitorous when the block happens; causing a person to take damage. For it to be traitorous, the following must be happening: A player has already jumped to a location they believe to be unblocked. The blocker moves into the area to block the jumping player. There can be no other place for the jumper to land safely. This pertains to all jump blocking scenarios; not just assault.