How to Stop a Cat Climbing Curtains
I'm going to suggest using a principle called classical conditioning to make your cat associate climbing the curtains with something unpleasant, such as a loud bang or other noise.
Classical conditioning has been used for many years in all aspects of life, for example you are bombarded with it every day in advertising. Its main use is to make either a human or pet associate a pleasant or unpleasant situation with a behaviour that you'd prefer to encourage or discourage. So by using classical conditioning we will make your cat associate the action of climbing the curtains with an unpleasant noise.
The most effective thing that I found train a cat to stop climbing the curtains is with a rolled up newspaper. Make sure that you have a newspaper handy in the area where they tend to climb curtains so that every time you see the attempting to climb the curtains, you will be able to bang the newspaper on a hard surface to make a loud noise. This should distract them from either climbing the curtains or if they're half way up the curtains you should be to get their attention and demand and in a firm, low voice to get down. If you find your cat ignores your request to get down, then you can hit the paper on something hard (definitely not the cat!) again until they jump off the curtains. This is your first step in telling your cat that climbing the curtains is unacceptable behaviour.
You must repeat this action every time your cat tries to climb or manages to climb the curtains, otherwise you'll be sending your cat mixed signals and you will not be able to train your cat to stop doing this behaviour.
You may find that you have successfully managed to train you're not to climb the curtains however your cat just like children will try to push your limits on occasion and you may find that they start to climb the curtains again. If this happens just bring out the newspaper again in the next time your cat considers climbing curtains bang it on a hard surface as this will quickly remind them that this behaviour is not acceptable.