Sounds like there must be some smell from the couch’s previous owner that is triggering marking. Because they are soft, they absorb smells and it will be really hard to completely remove the smell.
The most effective option at this point would be to block access to the couch with an x-pen for at least a few weeks. In time the bun might forget about it, or the “new/old couch smell” could dissipate on its own. Bunnies can really get into habits, so the more times he pees on the couch the more likely he is to continue.
Another idea could be to try cleaning the couch really well with an enzyme-based cleaner, such as nature’s miracle. You could really soak the cushions and let them dry.
There are also tons of waterproof pet blankets on the market these days. You could cover the couch with them (or another waterproof couch protector, they make lots of them for dogs), and then at least it would be easier to clean up.
I’ve also heard of some folks on the forum having luck with putting a litter box ON the couch. Not really ideal, but if it works it might be worth it!
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.