Users do not receive a notification if you reject their posts in the moderation panel, reject their request to join a group, or delete/archive their posts from/in the community.
This is not an intuitive user experience. Take Instagram and Facebook as an example. If you reject someone's friend request, the user does not receive a notification for the rejection.
However, in case you want to reject a content from the moderation panel, you can leave a comment under the posts, give it a few minutes, then remove the post from the community. This way, although the post has been removed, the user still receives a notification for your comment, explaining the reason for the post being removed.