I personally do not use fixative on my pastels because it darkens them, reducing the reflective properties of the individual pastel particles clinging to the surface. In effect it fuses them together as it fixes them to the board. Even so fixed they can still smudge, though.
However, posterboard is not known for having a good tooth for pastel and you may need to use a spray fixative. Be sure to follow directions, holding the spray can far enough from the surface (vertically) that you won't spatter it with the blobs of spray that sometimes happen with aerosol cans. Several very light layers, dried between, are preferable to a heavy layer.
"elaminate" is not a word and I have no idea what you mean by this. But if you are going to frame your pastel work under glass, be sure you use a spacer so the pastels don't touch the glass. The simplest way is to mat it first, and use strips of matboard glued behind the mat to lift it off the surface of the work. That way if any dust does dislocate from the painting it will fall behind the mat!
Good luck . . .
