It is almost impossible to make cats do what you want them to do, but it is possible to make cats not do what you want them not to do.
There is a hierarchy of actions which you should start carry out.
- Blow air to his face.
- Spray water to his face.
- Hit on the ground with a stick/club, like a meter or two from the animal.
- Start a vacuum cleaner near him.
- Chase him with a vacuum cleaner in your hand.
The first two actions will tell the kittens that you don't like their actions. While this would be enough for dogs, cats don't care if you like their actions or not. So, instead of stopping climbing, they would check if you are around to see them climbing and they will jump off whenever they hear you coming. This is where the last three actions come handy.
Cats are excellent judges in risk vs reward. The last three actions will tell the cats that there will be consequences. However, you have to do these actions while the acts are climbings, i.e., you have to catch them red pawwed. Otherwise they will not make connection between you reprimanding them and their actions.
The final approach should be be beating a puppet. Take a cat puppet and put it on net. Make sure your cat sees it. Then beat the puppet with a club or with your hands, right in front of your cat. Keep in mind that this is an extreme method, so I would suggest you to save this only for extreme behaviours such as drinking from toilet or wandering on the kitchen counter.