OK, I mean, you get back what you give out.  So, if you have a stern look on your face, look like you're all bothered by the interruption etc, then people will give you the same attitude back.  I smile, giggle a tiny bit, turn my head a little to one side and say, very gently and slowly with an apologetic voice "noooo, I don't work here, I'm just taking care of Revlon's products here."  and then I go back to work.  Usually they apologize, say something like "Oh, I thought you did because you have a tag on," or something in that vein.  I say, "I know what you mean, they make me wear this tag."  Get all humble and $|-|1T, don't maintain eye contact (cause that's a challenging stance lol) and you get 10x back to you.