Just make an event to apply an opposite force whenever the move keys are not pressed. If you apply the whole opposite force it will stop instantly, if you want a smooth damping multiply it by a float between 0 and 1 before applying.
Anyway. It seems clear to me that there are a couple of features missing to make a fully-featured Mario/Sonic style platformer:
- Families (duh!)
- Using events to selectively choose what a certain physics object can collide with or pass through...[/QUOTE]
You mean a (physics based) platformer, and that's not totally true.
Families is obviously an extremely useful feature for any game, but it's certainly possible to make things work without it, it's just more labor intensive.
In the same way it's possible to make a "loop gate". It would be really handy to have an action to activate and deactivate physics from an object through events, or even have selective collisions, but for now you can make an event to set the physics to "immovable" and move the "gate object" to another position. You can then place an identical object below the "gate" so nobody will perceive any visual difference.
Again, it's already doable, although it would be really nice to have some features to better support it.