Assertiveness-less

I definitively need an assertiveness (I don't even know if that word actually exists!) course.
Thing is, I'm in charge of an area here in my company, and there's a salesman who I'm in charge of.
He's REALLY stupid and looks in a sick-pervert way that gives me the creeps (in fact, once I dreamed that he tried to rape me!). No, he's not stupid, I think he's very clever actually, but in a wrong-untrustable kind of way, you know what I mean??
I think he thinks he's more clever than me, and that I can fall on his lies or "storytelling", but I know better.
He talks and talks and talks, but doesn't say anything at all, uses words that are totally out of context (but sound "cool"), he doesn't even talk well (it's like if in english he said: i'm to going to call you, something totally not right).
Well, as you can see, I'm not very fond of him, in fact, with my boss (my uncle haha) we're talking of firing him, because he's giving the company the worst image, and we've already had some complaints from customers.

What does this have to do with me not being assertive? Well, today I had to argue with him, because precisely a customer had called to say she wanted to be attended by another sales person because she didn't like this man. So I told him to stop visiting that customer. And he sent me a HUGE email telling me basically he was not willing to loose that customer, and all this full of strange words, that this project was like a "flame", that this was a road being pavemented and once it was ready, everyone could be able to transit along that road (of course we do not build roads!!!!), and bla bla bla...but it was in a kind of defiant tone that made me really mad, so I had to tell him, and he defended and I continued being mad....

But, the thing is I don't confront people, I get too shy when it comes to tell things to unknown people (for example if a waiter takes too long to bring my order, at the most I will say him with a big smile, "hey, is my food alright?", you know??), so this confrontation left me almost trembling. I feel proud of myself to had confronted him, but the post-feeling is not nice.
That's why I think I need a course that teaches me how to say things without being affected in such a bad way!!!