Ok so my son turned 2 in December and he starteed doing this at about 15 months. I just let him play then just made sure I woke him up at the usual time, and yes he was crabby but we always told him "well you should have gone night night when we said" and it took some time but even though he didn't fully know what we were saying we said it so matter of factly that he eventually caught on, because they catch on to "you" and "night night" and "we said" very quickly if your consistant.
He still plays instead of going to sleep at nap time but he is a little more apt to do what I say because he has begun to remember the not so fun part of not taking a nap (the crabbiness without the attention).
That is what worked for us, I use the same kind of thing with the school agers that I work with, it is just pointing out the consequences of their actions in order for them to make better choices for themselves.