@keshlam Unfortunately, ignoring them when they might be communicating work related stuff might not be so good. If weren't at work I'd say ignoring would be best.

That isn't an appropriate way to communicate "work stuff". Unless it is part of your job description you are under no obligation to accept information transmitted in Morse, signal flags, pheromones, telepathy, song and dance numbers, mime, whistle languages (yes, they exist), Latin, Pig Latin, pig grunting, pork products, pork byproducts... If they are supposed to tell you something for work purposes they are supposed to tell you in a reasonable business -appropriate manner. Their refusing to do so would Not be Your Problem.

I'm genuinely surprised that you take the time to decode it, in a way you're enabling its continuation. Just tell them to stop, if they don't just leave after the first word or cough to interrupt the message flow!

Same way you deal with anyone who keeps trying to speak to you in the ''wrong language''. You just ask them politely but firmly to go back to the other language and do not respond to the one they want to use. This has also worked with me when a person keeps persistently trying to speak to me in English in a non-English speaking country even though I know the correct language, just don't be over-aggressive with it. Don't wait until it gets worse as you might just explode with rage.

If I realised that two of my colleagues were communicating in morse code in a meeting, I'd be tempted to covertly record it to decode later, to find out what they were communicating about.

Strange! It is not possible to decode morse code generated by tapping on a desk because one can not differentiate between dots and dashes. There are no long taps.

Man, there are weirdos in this world... I would start responding with nonsense until he finds another guinea pig.

@Elec1 you quickly double-tap to send a dash. tap tap tap tap-tap tap-tap tap-tap tap tap tap

is his name Dwight maybe? Dwight Schrute?

"Please stop communicating with me in Morse Code. It's distracting and I'm finding it hard to concentrate" is polite.

@Elec1 - maybe he scratches for the dash?

Are you sure that you're not being inducted in to an awesome secret society? Check for that before you brush them off.

Reminds me of a friend who noticed two of his students tapping Morse during an exam. Being a ham himself, he tapped out "You've each just lost ten points on your grade." You can't get away with this in meetings, any more than you can texting each other. Possibly less.

"Hey dude, this is entertaining enough, but please stop."

What @Tom said about the similarity to speaking the "wrong language" is very good, I think.