Wash the area with biological clothes washing liquid. This neutralises the smell to deter them going back. Bleaches heighten the smell so cats will return to that spot. Also feed them next to where they have toileted. They don't like to toilet near food sources. Then find a hiding place for the litter tray as far away from the food and bed place as possible. Try using soil in the trays as if they are feral they will not know what litter is for. Finally, when you find a poo put it in the tray so they can smell where it is to tempt them to use the tray.
oh and don't wash out the tray for a week or two, empty it and replace soil, yes, but don't wash away the toilet smell until they have the hang of using the tray. Also make sure there are a couple of trays as some cats won't go where another has been.
Hope this helps and good luck. There's nothing quite like the stench of cat poo