If the boxes should not move, simply set them as immovable.
If they need to be moving, it may be difficult to get rid of that twitching. You can try experimenting with settings like elasticity, friction etc. Or briefly (for 0.1 seconds) set them immovable with an event, maybe this will calm them down..
If your main concern is the gap, you can decrease the size of collision polygon in the box sprite by 1px at the top of the box.