How do you proceed when people scream at you in the workplace ?

My gentleness, politeness and soft voice often make people believe that if they scream at me, the rules and regulations applied for [b]everyone[/b] will somehow be disrespected especially for them. I and the whole team get blamed even when clearly the client did not respect the procedures (already sent to them). I try to go by protocol asking them to keep a respectful tone but it's like .. nothing works. Do you know an instant way to make a screamer calm the heck down ? I am there to help but I just can't endure it anymore. I have to find a strategy to make them shut up and listen (it is hard to be taken serious with a soft voice lol).. any ideas ?