This is a pretty common problem, especially in multiple cat households. It is usually a territorial issue - which is why male cats are more prone to do it than female cats. The person that finds a good solution will become a millionaire - at the very least! For now, though, there are some options that may help you.

If there are any strange cats coming to your house, you can try a deterrent to chase them away. This is one of the most common causes. Even if you don't see them and your cats are spraying near doors or windows, you can be sure you have visitors.

Clean sprayed areas with an enzyme cleaner. I've had some success in the past with calming collars. Be cautious of prescription meds. I had one cat that had seizures possibly caused by them.

Good luck! Also, if you haven't seen "My Cat From Hell", I would recommend watching some episodes of that. He has some good tips that may help your situation.

For me, though, it has just been attrition - getting my cat numbers down after the spayers have passed on. The spraying also decreased as they got older.