We have two spayed females. We have a magnetic catflap (they have magnets on their collars to open it). Over the last few months an inneutered male has been getting in by using brute force and has been spraying in the house. It's disgusting. Once we find it we can get rid of it but it's not always easy to find, and it's so upsetting to come home to a stinking house.
Managed to find the owner via a local Facebook group. Told her what was going on:l. She was mortified to be fair, she said she and her daughter hadn't wanted to get him neutered
but now they knew he was causing problems hmthey would. The problem continues and got worse to the point he was coming in daily. Kept in touch, and he was neutered last Wednesday. Had a lovely few days of a non smelly house, but today we can home and found him in the house, and he'd sprayed 
I know neutering doesn't always work especially when left until they're older. What can we do? Over the last few months we've had so ray on our curtains, the kitchen surfaces 