If you DON'T follow this advice, you'll be on the right track:

Ignore their needs
Nothing says "I don't care" quite as much as simply not being there when someone needs you. To achieve this goal, there's nothing you need to do.

Don't acknowledge their accomplishments
People want feedback; even if it's not the most favorable feedback, at least they can use it to grow. This is a variation of "ignore their needs" because this is something that they look to you to validate them in some way. Your opinion, for some odd reason, actually means something to them... so by not acknowledging them with a good word or constructive criticism, this one can actually hurt more than just ignoring them.

Tell them they're bad/wrong
You could actually go out of your way, if you are really into hurting someone else. Just tell them that you think they're wrong for being who they are. If they care even a little bit about themselves, you'll get them to become angry as well as hurt. If they have very low self-esteem, this will be like kicking them when they're already down.

Change the rules AFTER the fact
This one is for those who actually are trying to participate in the lives of other people. For those who don't want to ignore them, but simply want to de-motivate them through confusion, simply change the rules AFTER they've accomplished something. Sure they did what you originally asked them to do, but now they're wrong. They should collapse under their own frustration and perhaps quit entirely.

Break promises
This one might make you look bad, since you're the one who's doing it; but as long as you don't mind that, feel free to not follow through on what you promise. Sure, they'll eventually learn not to rely on you, but you can still dash their hope now.

Make excuses
This often accompanies breaking promises. It tells the person that something else is more important than they are. If you come up with some really good ones, they might even believe you're doing it in "their best interest." Good one! It makes THEM feel guilty for you not caring.

Too little/too late
If you really care, but still want to de-motivate people, you can try giving a little bit, but not enough to be of any good. "A day late and a dollar short" still prevents others from reaching their goals, but at least you can feel good about yourself.

Live their life for them
This is for those people who really like to go to extremes. You care so much that you don't let someone else do anything for themselves. Meddle in everything! Tell them what they should and shouldn't do; even do things for them that they don't ask for. This way you can be the martyr and they can be the ungrateful ones. This actually makes them look like they don't care about you. Talk about "turning the tables." This one is a true classic.

So you see, it can range from zero to over-caring, but you can de-motivate people at every level. It doesn't take that much effort at all.

Caring for others, especially when it comes to encouragement, takes a lot of work. You don't have to do it; only unless you want to help others become better than they could on their own.

You have more power than you think. It all depends on the results you want... and if you really care.