What my sparring instructor used to tell me:
"worry about your accuracy first... think about correct stances and focus on your target. it doesn't matter how hard or fast you hit if you miss your target. slow down, relax, and learn the correct motions...
next, worry about your speed... it doesn't matter how accurate you are, or how hard you hit, if you're so slow that they can just move out of the way....
finally, worry about your power. with accuracy and speed come power. The harder you try to hit someone, the more likely you are to be off balance, or in an improper stance. make sure your power comes from the right places, not from sheer muscle, because there's always going to be someone out there who's stronger than you, so muscling through won't always work."
They need to learn that the martial arts is about more than just hitting hard. Its about developing proper body alignment and speed, and with the correct accuracy and speed, power will come.