bad clients

Hello All... I have recently encountered a problem at my work with my boss and I am worried if I handle it wrong I could be fired and I could really use the input of this forum. First I must set the stage. So, I work in a biotechnology firm as a financial analyst, but sometimes I dabble in lab...

How do you guys usually deal with problematic clients? Seeing as how we freelancers are working for people all around the world, there is definitely a good chance to have a couple of "bad" clients.