I don't see where you are setting the player animation to "Climb" anywhere. Did you delete it from event where the player is overlapping lader_climb? (I would think it would come right before or after the setting X action.
Anyway, here is a sample that I was playing around with. When the player overlaps the ladder the platform behavior is disabled and he starts climbing. When Space is pressed if the player is climbing then the platform behavior is enabled and climbing is turned off. So there is no dismount area. The player can dismount anytime and suffer the consequences. But it does stay put on the ladder.
ClimbingTest.capx
Note if player is red default animation is playing, if purple climb animation is playing.
EDIT:This was created with R83, so you would need to update C2 if you haven't already. Here are my events:
<img src="http://dl.dropbox.com/u/57899112/ClimbTestEvents.PNG" border="0">