I do sympathise as nothing worse. I have an entire girl who is prone to spraying the cushions on one sofa!

First of all, if you know which cats are doing it then I would get them checked out at the vets to eliminate any UTI problems.

I note you said you have plenty of litter trays - which is great but have you considered trying a different litter in a couple of them. If you are using a clumping - shift to a non-clumping or maybe recycled paper or wood based one. It may be the cats making the mess just don't like the litter you currently use and look to go elsewhere.

Are all your trays open - if so, perhaps buy one hooded or vice versa.

You could try a couple of feliway diffusers (plug ins) around the room to see if that helps.

Good luck.