If swallows are in the area, it is important to keep an eye out for any potential nest building activity. As soon as nest building starts it is important to hose down any part built nests, making sure that all the mud is removed, otherwise they are more likely to rebuild.
However, once swallows have identified a potential nesting site, they can be quite persistent in their desire to build a nest. As such using a professional pest manager to exclude the birds from the area becomes the only option.