Report flagrant multiboxers?

Q u o t e:

As others have said, simply multi-boxing - as in running multiple accounts at one time and controlling the characters on all of them - is not against the rules or policies Blizzard has in place.

Now, to add onto that, botting and hacks are still against the rules. Multi-boxing as done by having a single keystroke sent to 5 different clients is fine. Having that single keystroke make each character perform multiple actions is not. If the multi-boxed characters seem to be able to do things that a group of 5 very coordinated players could not do, you could submit a report on suspicious behavior. (Think a group of 5 friends at a LAN party calling out and syncing their movements)

Indeed, and there has been exhaustive discussion on this topic in the past. To end another discussion in this thread: multi-boxing is an unsupported play style. For now no actions have been taken specifically to prevent a player from multi-boxing, and as long as the practice remains within the bounds of our policies, that should continue to be the case.