Sprinkle cornmeal under the food bowl and where you think they are coming in. I have feral cats that I feed outside. It takes about a week or so, but eventually they take the granules to their nest/colony and die.
Are your ferals altered? I don't know where you are, but hopefully theres a low cost clinic
in your area. Tons have sprung up recently. You can borrow a trap from your local humane society.
Good luck and thank you for feeding them!