My local farm shop has allows allowed the swallows into their barn. This year, I noticed they are trying to stop them. Some of the birds have nesting materials in their beaks, so I know they have at least built a nest, they are flying around rather frantic, but nobody is interested.
You need to very certain that the Swallows have actually started to build nests rather than just flying in to a known designation in the hopes of starting ... some photos would help! If the owners are actively interfering in already started nests being completed then they could be breaking the law but ... there are exceptions such as health & safety reasons for deterring the Swallows etc ... if you are sure of your facts then you could contact your local wildlife officer at the Police Station and get expert advice!
2013 photos & vids here
eff37 on Flickr