Barn Swallows and occasionally Cliff Swallows frequently nest under eaves of houses, where the exterior walls provide a firm surface for them to attach their mud nests and the eaves provide protection from the elements. Since it is illegal to directly interfere with birds that are actively building their nests, it is too late to legally stop them when they show up and start building their nests. The best way to discourage them is to either block their access to the area with netting, or to make the walls unattractive to the birds—either by mounting spikes or other barriers there, or smooth sheets of wax paper or other smooth materials that are difficult for the birds to attach their nests to. For more ideas on how to discourage swallows, see this fact sheet from the Alberta Ministry of Agriculture and Rural Development.