I grabbed the trial off their site and tried it using .Net 1.1 (VS 2003). It doesn't visually change the PopupButton but it removes the onclick handler of the hyperlink so it won't do anything when clicked. I bet you could add code in these functions to swap in a disabled button image or something.

Note that this code is manipulating two (actually more) different client side html controls/objects generated by the server side calendar controls.

DateInput is a hidden html input control that they appear to use behind the scenes and pass any changes (like calling Disable) via JS onto the separate visible input control.

PopupButton is a hyperlink that pops up the calendar when clicked. This code removes and reattaches the onlclick event handler to enable/disable that.

Yes, it would be nice if it did all this for you with one call to the object represented by the main ClientID (also a hidden html input control BTW), but it doesn't.