When you say something controversial or joke about a dead person, there's always someone somewhere who'll get offended (the more truth in your statement is, the bigger the outrage, usually). Too many people still don't understand that someone's words, someone's joke doesn't necessarily have to affect them, that these thing can be safely ignored without any negative repercussions. That said, I understand that there is an aspect of "too soon", that some things are better left unsaid for some time, out of respect for those in mourning.
As for the phrase "never speak ill or joke about the dead", if you want the user to stop saying this simply remind them of Adolf Hitler. That, in my experience, tends to shut them up very quickly.