How to stop 3 male neutered cats from spraying everywhere?

My parents have 3 male cats, all neutered, who are currently driving them crazy by peeing up everything in sight. It's gotten so bad that they have to keep the cats from coming in the house because they have been going straight in and spraying up walls and objects. It seems to be worse at this time of year.

Is there any reason why the cats would be doing this and is there any way to stop it?

What is a VPN?

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