Santa and Shut up

The Hulk has taken to saying "Shut Up!" constantly.  It's usually the first thing that comes out of his mouth when he wakes up.  I don't know where he got it from, why he keeps saying it, or how to stop him.  I've been saying "Santa Claus" in reply, hoping he'll make the connection that Santa doesn't like to hear little boys saying shut up, and Alex knows it's a bad thing to say.  But this approach hasn't stopped it yet.  I think the best solution so far has come from his older brother Calvin.  He told the Hulk to stop saying it because Santa may pass over the house all together and neither of them will get any gifts.  For goodness sake this may impact him!  I like not having to take this one on.  Go Calvin!