In this scenario, We have a distribution group and it accepts messages from all sender. I would to like reject messages from one user.
Open Microsoft Exchange Server management Console and go to
Microsoft Exchange On-Premises>
and choose your distribution group that you want customize and right click on it and open properties and click Mail Flow Setting
Open Message Delivery Restrictions
and then click “Sender in the following list” and click add and find your user who needs to be blocked