You can always retroactively delete the callbacks in the property inspector and in the generated MATLAB file.
Possibly this is what Sean was already implying, but I just discovered that if you delete code for a callback completely from the generated mfile, they won't come back when GUIDE updates the mfile with further changes. To restore a deleted callback, you just have to use the uicontrol's "View Callback" menu option.
This is ultimately what I was looking for. I had assumed, incorrectly, that a GUIDE update would always regenerate callbacks in the mfile for all uicontrols in the GUI.
+1 to Sean and thanks to everyone else.