Don't make a policy then not enforce it. The ones that I have seen most frequently are designated smoking areas and using "boomboxes" in public spaces.
Even as a smoker, I understand the need and desire for smoke free spaces, even on the outside decks. Please, when someone is smoking in a non-smoking area - make them leave or put it out. It is just as much a right to non-smokers to have a smoke free area as it is for me to make my own decision to smoke.
When someone is walking around with a "boombox" or on the Lido deck by the pool with a loud radio - let them know that the policy is to use earphones. Not everyone may want to hear your music.
And the one that I think gets most people riled: Chair saving. If it has been an hour and all that is on the chair is an unused towel and a book, then remove it for those who plan on staying for a while. Those who left it can pick it up at the purser's desk