We usually move him away and ignore him, this was disturbing people so I strapped him in his car seat (rear facing so he couldn't see me). Then I patiently explained that he couldn't have the music on because we weren't there for a treat!
Ah well, the 2 incidents will go down well when I'm back at work tomorrow!
And when I did "difficult conversation" training at work I was told I stayed amazingly calm- I was just using the techniques the DC have taught me....