Well, by design, you should not let the reject the records and let them go to sink. Rather, control the logic for rejection, so that in future if there is a change to that rejection logic, you have control in ur hands.
Further, the rejected records are logged to session log by default, since its supposed to be abnormal behaviour on a mapping's part that some data is not handled properly through the flow.
To avoid getting all that data logged into session log, you can change the tracing level of the session to Terse. But remember, in that case, you wont get lot of other logging information in session, that is generally useful. This will achieve the end result, but is not the "ideal" way of achieving the same end result.
I would suggest looking again at your mapping design.