Incompetent but cunning collegue is often successful in dominating conversation.

I work in a sell side research/media company. After failing to convert 1 year internship into a permanent role on a trading desk and then getting fired from another company I had to settle in a more relaxed environment. But I now find myself in a situation where I have to deal with a cunning team mate on daily basis. This dude cant do basic tasks but somehow he has convinced people around him that he knows this specific commodity narket. He takes advantage of the situation that the third person  does not know about our market  so whenever I am talking he would just disagree with me and try to dominate the conversation. He has talked behind my back and sort of made few ppl question my abilities. He has mostly been successful in making other people believe that I am weirdo but he cant prove that I am not a decent analyst. The fact that he is the most incompetent in a division of 25 employees but can talk over me in conversations pisses me off. This is the first time in my life I have felt that 48 laws of power is actually a decent book. Because that asshole is definitely implementing few rules very well. His 2 best tricks are.....1) Talking shitt about me behind my back but maintaining good rapport with me. 2) Being a typical nice guy and befriending everybody. 

I am leaving the company as my contract ends but I cant let this asshole ruin my reputation once I leave. His one weakness is that he always becomes friend with somebody by bitching about third person. At some point ppl should realize his cunningness and incompetence but its strange that in 6 months nobody has been suspicious of him.