Make sure there’s no problem. Although you don’t want to reward meowing, sometimes cats meow for good reason — they can’t reach their litter box, they’re trapped in a room, the water bowl is empty. Check on them when they meow to determine if it’s something you can safely ignore, or a problem that must be corrected right away.
Don’t punish a cat for meowing. Hitting, shouting, and spraying cats with water rarely work to quiet a meowing cat in the long run, but all those actions will make your cat distrust or even dislike you.
Don’t give in. If your cat is used to getting what he wants from meowing, he’s going to meow more, and louder, when it quits working. In other words, it will probably get worse before it gets better. Just keep rewarding quiet behavior and ignoring meowing, and eventually he’ll get the idea.