My suggestion would be to snatch the cats and get them nuetered. With the amount of money you are going to spend on cleaners you could have them both fixed and be done with it. I know they are not yours but if the owners don't look out for them they may not notice them missing. With males it isn't as big a deal and if you can find a low cost spay/nueter clinic you could just do the TNR (trap, nueter, release). It sounds like the owners would never know and even if they did figure it out they wouldn't know who did it.
I know this may not sound like the greatest of ideas but it is just going to continue and it will be very hard to get rid of the odors outside.

GOOD LUCK!