We used to have "friends' who would bring an uncontrolled dog when they visited, so after talking it over, as soon as they were seen coming up the walk (they never called first, probably because they discovered no one home") I would call one of ours into the house and put him or her on the "down, stay" and let him lie there, but never corrected him when he snarled at the visiting dogs. Folks never seemed to want to stay long after that.
All ours were rescue Dobermans that I had schooled myself.
Lovely beasts, better behaved than some of my one sister's kids.