cancel a call to impoly

All,

I'm manually outlining a series of ROIs on some 3D data, and would like to know how to cancel a call to impoly (i.e. when a user has optioned to stop drawing) by way of a togglebutton.

Fundamentally, after impoly is called, I can't seem to find a way to break out of the call until AFTER impoly has had some input (i.e. something is drawn)

A simple example:

 r=imread('cameraman.tif');
 imshow(r,[])
 h=roipoly();

After all this has been entered, how could I cancel the fact that I've called roipoly? Its clearly waiting for input, perhaps through waitfor? I've tried deleting the handle and uiresume with no luck

Thanks

Jim