I was told to try this with guineas, and didn't, but have tried it with chickens and it works like a charm with them. Still haven't tried with guineas, but I intend to.
Get an umbrella and when they start to go where you don't want them to go, (if you can be there when they go there), take the umbrella and open/close it as fast as you can. Hubby said, and I agree, that it sounds like a HUGE bird flapping its wings.
My chickens were getting on my porch (of course they did - - I was feeding them treats on the porch like an idiot, but live and learn) and leaving literally 30-40 poop piles everyday. I was cleaning the porch every day. I tried the umbrella and after two times in one day, it worked beautifully. I did have to reinforce the training once after about a week, but now they avoid the porch at all costs.
It just might work with guineas. I hate to frighten them, but it's better than the stewpot or re-homing. (Hide your face behind the umbrella so that maybe they don't know it's you doing it? He!!, I dunno.) I hated to take an hour a day to clean the porch. Now I don't have to.
Moral: Try everything within reason - you never know what might work.