2 options:
Use the getproperty action to determine the x/y position of your dragged mask, along with an if statement containing a setproperty action to lock the position if it crosses the target location.
or, probably better...
On your draggable clip set a variable (say x=1) when the drag is commenced. Create a transparent button in the required target location with an action which (on rollover) firstly stops the drag if x=1 and secondly sets the x/y positions of the movie-clip to the required location.
dD davdesign@hotmail.com
^^^^^^^^^^^^^^^^