not let catch doing

not let someone catch someone doing something

 and not want to catch someone doing something
an expression that scolds someone who has done something wrong. (The idea is that the person ought not to do the wrong thing again, not that the person simply avoid getting caught.) How many times have I told you not to play ball in the house? Don't let me catch you doing that again. If I've told you once, I've told you a thousand times: Don't do that! I don't want to catch you doing it again!
See also: catch, not