@berwin -

in the assignment group where you are dropping the lowest score, you can also select an assignment to never drop (from the same edit spot).  the only problem with that is that it will apply to all students so some students may not get their lowest score to drop.

it would be nice if you could select which students that applied to, but that is not possible currently.

another option you could have (you will have to try it out and see if this works).  Create another quiz assignment for those students that cheated (differentiated assignment - just assign it to them).  The other students wont see it - only the students that you assign it to.  Then put it in as the never drop assignment, excuse them on the original and then see if it drops the correct score for those students.

edit - the above seems to work.  Create a differentiated assignment for the students that cheated, excuse them from the original and then add the special differentiated assignment to the don't drop list.