|This page in a nutshell: When under the accusation of dickery, if it is clear that the action is uncivil, readily cite WP:MOBY as a riposte.|
One of the easiest things to forget while editing and one of Wikipedia's most important aspects is that this is a medium used by people from many different cultures and societies. So don't get like Moby Dick.
Calling someone else a dick
Civility is one of our most important guiding principles when interacting. And in many cultures and societies being called a "dick" is profoundly uncivil. If you do indeed believe that someone is behaving badly, behaving badly yourself is not going to win you the moral high ground, much less the argument.
It's also worth bearing in mind when considering calling someone a "dick" that many ambiguous usernames disguise female editors.
When you are accused of being a dick
Having WP:DICK thrown at you is quite difficult to deal with. Protestations that one is not a dick do not calm the situation and, worse, move the focus of the discussion away from the matter in hand to focusing on your own behaviour. Ignoring the accusation seems to tacitly agree that your behaviour has been poor. And either option overlooks the reasonable objection that being called a dick is uncivil.
If you, or for that matter someone else has been accused of being a dick, you can respond by simply pointing the other party to this essay:
That's it. Have a whale of a time.