Bubba -
It depends on how the diaphragm is modeled. If you are using the "rigid diaphragm" feature in RISA then you would do as you suggest and select all the joints in the hole and designate that they "detach from diaphragm". The only problem with this is that the RIGID diaphragm is very rigid as the name implies and the diaphragm will still behave rigidly... it just won't be connected to those joints.
If, however, you are more interested in the forces in the diaphragm in the vicinity of the opening then you would probably want to model the diaprhagm using plate elements. That way, you can see exactly what forces occur around the opening.