If you do not mind blocking also the keyboard you can use

declare integer EnableWindow in win32api integer, integer

EnableWindow(thisform.hwnd, 0) && Disables all input
* Your long process
EnableWindow(.hwnd, 1) && Re-enables it

But I am not sure if this will leave the calls in the stack, another alternative would be using BlockInput

declare BlockInput in win32api integer lBlockIT

BlockInput(.t.)
* Your long process
BlockInput(.f.)

I understand that blockinput will not leave any calls in the stack, but the side-effect of not even being able to move the mouse is ugly, and I think BlockInput should work, I just run a small test