I know how you feel. This happened to me on many occasions. I'm sorry people couldn't appreciate the full power of that organ but this happens regularly at many congregations after the service - people want to talk to each other.
What can you do? Nothing - really. At least not what you think would be fair - crank up the volume of the organ no matter what. Just like you can't force somebody to read your spammy email.
Yes, you can try to trick them with subjects, like "urgent" or something between the lines of "ignore this and you face the consequences" but you know what will happen next if you do.
The Delete Button. They can press the delete button when you play, too, you know.
People choose to pay attention to the things they like, know and trust which is usually themselves. Remember, they didn't come to listen to you play - they're having a good time talking to their friends and you just happen to play the organ there (or so it seems).
But sometimes, if they are the right kind of people, you can choose to earn their trust too.
That's when the magic starts to happen.
[HT to Steven Monrotus]