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I did that already... but the host of the contest isn't that fair either because they themselves used a picture of the brand we spray with as well, so i think they all went to my page and took what they liked... :(

I don't use instagram but, doesn't it have some function that can report it to get removed like facebook?

It does, but most of the times your request is not followed up... at facebook as well, you report it and you get 3 options: ask the person to remove it / don't follow the person anymore / block the person so you don't see his or her posts anymore.. so that's not really a solution...