How do I Lock the translation in one axis?

Hi,

I'm trying to lock the movement of an object when I'm using drag and drop and physics. I tried different ways but I always get unexpected results.

This is the file I'm working on:

drive.google.com/file/d/0BzLq20Y55riOQUYwTGVkV0VpSXM/view?usp=sharing

Any help is welcome.

Thanks.

Drag and drop has an option for working in a certain axis

Thank you