Dog scent marking house

A work colleague rescued a male JRT about 6 months ago. He is about 2/3 years old and was castrated approx 4 months ago.

He has a few 'problems' all of which she is working through but he scent marks in the house and she does not know what to do to stop it.

I haven't had any experience with this so I am unable to help her - can anyone give me any advice to pass on to her please?

He will come back from a walk/in from the garden and raise a leg so it's not because he needs to go.

Thanks

Castration is usually the answer, but with an older dog, they`ve probably already got into the habit. She needs to clean the areas to get rid of the scent - he`ll be doing it in the same places - using a non-ammonia based cleaner. I put Dettol down as well as they don`t like that smell.
Maybe keep a closer eye on him? And distract him when he goes to the usual spot?
A friend of mine stopped her rescue doing it by walloping the marked spot with her hand shouting Naughty Naughty Wall. (In front of the bemused dog). It worked, though I have no idea why. I can only imagine the dog assumed it was `her` wall so left it alone.