How can I teach a dog to stop chasing cats?
How can I teach a dog to stop chasing cats?

Step up and be more proactive. Put a leash on your dog when the cat is around. Reward him for ignoring the cat, prevent him from chasing the cat. Don't give your dog the opportunity to chase the cat, period. Teach him a good leave-it command, but you don't start teaching leave- it with a highly desirable object like a cat. When you aren't there to supervise make sure the dog and cat are safely contained away from each other, otherwise you could come home to a dead cat. The more opportunities you allow for the dog to chase the cat the harder it will be to stop.For leave-it simply hold a boring piece of kibble in your hand, close your fist, say leave-it once and once only. Don't move your hand and wait for him to give up trying to get the kibble. When he does say Yes (to mark when he stops) and give him a high value treat from your other hand (do not use the higher value treat to bribe him away, keep the high value treat behind your back or even up on a shelf).

He does not get the treat you told him to leave-it. You will find that with 3-5 reps of this he'll start to get it. Work up to higher value items, work up to leaving something that is on the floor. Make it harder and harder as he's getting it and finally work up to leaving the cat alone.
Raw fed agility dogs
Shylo, Cheyenne, Lakota, Shylo
How can I teach a dog to stop chasing cats?

Lauralyn already have some good serious advice, so I'm going to take off on a tangent with a funny about one of my past dobes, Atlas.

Atlas loved to chase cats. He made a big show of chasing and barking, but he never hurt them. Upon catching up to a cat, he would grab it around the middle (legs and tail out one end of his mouth, head and more legs out the other), and bring it to us. Once he'd been acknowledged, he'd drop the cat and the cat would run off.
Worrying that this was probably traumatizing (or at least really irritating) the cats, we worked hard with Atlas to teach him to ignore the cats. When that was finally successful, one cat would run up and push on whatever part of the dog it could reach with its paws, then run away. (If the cat could talk, it might have been saying "tag, you're it!") It seemed to miss the attention!