Ants invade a porch for three reasons: there is food there, it is a path to food or it is near their colony. You may not be able to do anything about the source that is attracting these pests to your porch, but you can keep the ants from continuing to come on the porch surface. Once you've applied the treatment, the ants will back off for some time. Wind and rain will dilute the treatment, so you'll need to reapply it throughout the year to keep the ants away.
Sweep the porch and remove anything portable that's attracting the ants, such as a garbage can.
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Place ant baits near the colony or on the porch. The ants will carry the poison into the colony and kill off the population.
Follow the ant trail to find the path they're using to invade the porch. Sprinkle black or cayenne pepper along the path. Also, sprinkle the black or cayenne pepper around the perimeter of the porch and on the ground surrounding the porch. Pepper agitates ants' senses, so they stay away from it.
Spray glass cleaner on ants when you see them on the porch; this will instantly kill them.