With normal use of ezmlm-idx, the sender of a rejected post is notified that the post has been rejected and if the moderators chooses to comment, the sender receives this comment, usually describing why the post was rejected. This ezmlm behavior cannot be disabled at run time.
If post are neither accepted or rejected, they time out. ezmlm-clean(1) notifies the sender when this happens. This behavior can be disabled with the ezmlm-clean(1) -R (not return) switch, which has to be placed on the command line of all invocations of ezmlm-clean(1) (normally in DIR/editor and DIR/moderator). If you for some reason do not wish to inform the sender of your editorial decision, you can use this switch and let undesirable posts time out, rather than actively rejecting them. IOHO, it is better to be "above board" and use the normal notification mechanisms, together with active rejection and informative rejection comments.
The ezmlm-make(1) -u switch uses moderation in a slightly different way. Here, posts are restricted to subscribers, but posts from non-subscribers are sent to the moderator(s) rather that being ignored. This to help the subscriber that posts from an alias of the subscribed address, or the occasional non-subscriber. In this case it is perfectly acceptable to just ignore non-accepted posts. Thus, using the ezmlm-make(1) -u switch configures the ezmlm-clean(1) invocations with the -R switch.