Thinking about someone or something you don't want to is so frustrating!
Ironically, posting this question is reinforcing the fixation.
The best advice I have is to attempt to change focus. Instead of focusing on not thinking of this person, begin to attempt to focus on new, positive things instead. Spending less time reinforcing a memory over time will eventually dull the overall memory and emotional impression you have of this individual.
Instead of removing something, you need to add something so there is less and less room for what is undesireable.