Summarize the following:
Sometimes you can't put a physical barrier up to keep your cat out of a room. In this case, you will need to make the room uncomfortable for it to be in. Make loud noises whenever it enters the room or shoo it away. Note that if you do this, the cat may associate the noise with you and start to avoid you.  You can even employ stronger methods in some cases. For example, spill a small amount of water on the floor of your bathroom if you don't want your cat in there. A cat may hate to get its feet wet on any excess water on the floor. Another example is to make sure the cat has nowhere to hide in a room. Sometimes a cat likes going under beds or in corners to feel safe. With this type of cat you can block off access to under the bed or any other hiding spaces. This will make your cat uncomfortable in the room. Every time it goes near the room, have a spray bottle ready to spray a little water at it. This will make sure it knows that its actions are unacceptable. However, only use this method if you are prepared to accept damaging your relationship with the cat. It is highly likely the cat will associate the water spray with you, the sprayer, rather than the room. Thus, the cat is more likely to flee and not want to spend time in your company. You simply place the device in the doorway to the room and let it deter your cat from the area. Put a small amount of vinegar at the room's entrance or in areas of the room. This usually helps, as most cats hate the odor of vinegar. This advice is anecdotal, however, as some people find it helps while for others it makes no difference. Alternatively, fill a spray bottle halfway with vinegar. Fill the other half with lime or lemon juice. Spray at the entrance to the room and on beds and other places the cat usually goes. This will deter many cats from scratching on furniture or spending time in the room. If you go this route, the vinegar spray needs fairly constant reapplication. Make a different room more comfortable for the cat. Make another room cat-appealing so the cat will go there instead. Offer the cat several nice sleeping places (although it's likely to chose its own) including a high perch. Provide food, water, and a litter tray, as well as toys. This involves making a big fuss of the cat and making being in the room you want it to be in a pleasant experience. The goal is to make sure your cat associates the room with good things and wishes to return there. You may want to scatter extra tasty treats around the room so that it seems a more appealing place.

Summary:
Make the room physically uncomfortable for the cat. Spray the cat with water from a spray bottle. In addition, there are commercial electronic cat deterrents that will spray a cat with a burst of air when it approaches the room. Place smells in the room that your cat doesn't like. Redirect your cat's attention. Use positive training.