When I moved back in with my parents a long time ago, my dad hated cats and wasn't too thrilled that Tubby was joining the household. Evidently Tubby pickedl up on this because the only time Tubby has ever peed outside the litterbox is when he peed on my dad's side of the bed. Not once, not twice, but 3 times. I was told either make him stop or he's gone.

What we did was spread plastic over the bed. Tubby didn't like the feel of the plastic on his little footsies so he wouldn't jump up there anymore, and even if he had the plastic would have caught the pee before it soaked into the bed.

Put something on the couch - tin foil, plastic, contact paper with the sticky side up - anything that Katie will not like. Your problem will be solved. Yes, you'll have some ugly something or other on your couch, but isn't that better than having cat pee in your couch?

Not being judgemental here, but throwing her outside is not solving the problem, and will only cause further problems like Moosmom mentioned. If you absolutely do not want her anymore, please try and rehome her. Behavioral issues like this will oftentimes disappear in a new home.