Tell them to have a Merry Christmas! or whatever the current holiday is.That ottadoit!
You could really drive them bonkers with this if you went all out. You could get one fo those books about Catholic holidays and Saints' Days, and if you think you'll run into a JW on a certain day, check out what Saint's Day it is and what greetings and customs are appropriate for that day, and upon encountering a Dub, use your newfound knowledge as though you've been observing the holiday.
And I imagine referring to their meeting as "Mass" or "Service" would annoy them, too.