Treating aggression in cats involves identifying the cause of aggression and tailoring a treatment plan to the type. For inter cat aggression, re-directed and territorial aggression, try to prevent cats that do not like each other from interacting. Have all cats spayed or neutered as this helps prevent fighting. If the cats are all sharing a smaller space you may have to use some things to calm the cats. Conventional treatments would involve using pharmaceuticals like clomipramine to calm the cat. If your cat is on a medication like this, he or she needs to be monitored frequently for liver and kidney issues.
From a natural perspective there is a pheromone spray called Feliway which can be sprayed around the house to calm the cats. It is also available in a plug in infuser for larger areas. L-theanine is a natural substance that can be used for cat anxiety and aggression. Herbal calmers such as Matricalm can be very helpful and can be used along with conventional medication. Chinese herbs and acupuncture can also be used for feline aggression. One caution about herbal medications in cats is that many are bitter and cause salivation. They often need to be mixed with something tasty like salmon or tuna oil, or put in pill pockets. As a holistic veterinarian I always look at the diet the cat is consuming. Many times switching the diet to a homemade natural diet is extremely helpful for any type of aggression.
Fear induced aggression and pain induced aggression can be dealt with by dealing with the root of the problem. Cats which are very fearful may need something like rescue remedy or a homeopathic medication to help with fears. Pain needs to be identified and dealt with either with conventional pain meds for cats, surgery or whatever is needed to treat the pain. Alternative therapies for things like arthritis or back pain can include chiropractic, acupuncture, herbal pain meds or physical therapy. Pet Wellbeing has an herbal medication for cat arthritis called Old Timer for Feline Inflammation.
Play aggression is common in kittens and young cats, however it can carry on into adulthood if the cat did not have a chance to socialize as a kitten. If your kitten has this problem consider getting another kitten. Do not let the cat play aggressively with your hands. Provide a lot of toys that the cat can play with or chase. If necessary give the cat a time out from you by putting him or her in another room. Do not treat the cat roughly or hit him. This may make him more aggressive. A spray bottle with water can be used to spritz the cat and interrupt this type of play aggression.
If your cat has this problem, you have a lot of options. It may take some patience but it will be worth it in the end.