It can take a little while to get used to new places, but part of it is also training and what the family does to help the dog learn to relax there.
My suggestion is that instead of jumping to legal measures, you leave a note on the door to let the people know that their dog is barking. Some people don't even know until their neighbors complain, because the dog doesn't bark until they're gone.
Recommend that they see a trainer for help and be patient with the owners of this dog, if you can. Training can take time.
Only good things,
Grisha
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Grisha Stewart, M.A., CPDT, CTP
Owner of Ahimsa Dog Training http://DoggieZen.com
Author of "BAT for Reactivity"
902 NW 49th Street
Seattle, WA 98107
Store Phone and Fax: (206) 706-0375
Phone when store is closed: (206) 364-4072
Award-winning training by professionals who care about people and dogs!