I suggest getting several scratching posts and some catnip and put the posts near the spots that it tends to scratch. Occasionally rub the catnip on the posts and it will attract the cat once again, the catnip smell wears off after a little while. This is what we are doing right now. I have a 9 yr old cat who doesn't scratch but our 3 yr old does. We just got new furniture so we really started working with Sophie, the 3 yr old, on not scratching. She has started going to the post on her own and has not yet scratched the new furniture. If the cats starts scratching the furniture tell it no then take it to the post and puts its paws on it to scratch. Ours would scratch the ends of the couch so that's where our posts are now.
You may even want to get couple different types of posts to see what the cat likes best. They have them that are covered in rope and ones covered in carpet(our cat doesn't like this one) and some that stand alone and some that lie on the floor and some that hang on a door knob.
Also putting double stick tape on the area where the cat scratches keeps the cat from scratching it too. They don't like the sticky feel on their feet.
http://www.drsfostersmith.com/Produc...09&N=2002+2017 you can check this out but I've heard that in craft stores they have big sheets(the size of paper) that are sticky on both sides, I haven't found any yet though. You may have to wait until the cat is scratching to do this though to know where its going to be prone to scratch.
You can definately train them though. Ours know not to get on the counters and tables and what rooms not to go in. The water gun sounds great, I haven't tried them but I've heard great things about them. Just get several and put them throughout the house so they are always handy. Good luck.