In my personal response I wouldnt do anything. Just because you were flipped off doesnt mean that you have to retaliate. Sometimes no response is the best response. It shows that youre more mature than that and can handle situations well.
Im not saying to allow youre coworker to walk all over you. But in this case noone was hurt & its best to leave things as they are. You clearly see how that person feels about so youll have to stay out of each oters way. If it comes to a point where the coworker is constantly bothering begin to collect evidence and report to the boss or deal woth the matter in a different way. :)
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