This always puts a smile on my face (as long as they don't get insulting/vulgar). They are usually like: "I reported you!"
"Ticket is out!" and I go then like "Whatever... Do as you please" after I told them something along the lines "It's called multiboxing, google for it, go to dual-boxing.com for more info... and yes Blizz allows it, it's not against the TOS"...

I must have had quite a few reports by now, but I never have been bothered by any GM (not even this sort of check/inquiry for boting). Nothing. Nada. So I am rather amused (yet polite) by now with such encounters