Duplicate the clip needing the blur and stack it directly above the original.
add matte shape. select the size/shape you need. use the center controls of the filter to position the matte. Keyframe the matte position and size as the subject moves across the scene. ABSOLUTELY DO NOT USE THE CENTER CONTROLS IN THE MOTION TAB. You MUST use the center controls of the filter.
Add a gaussian blur, and crank the settings for desired effect.
Tweak the size/shape and position as needed. If the edges are a little harsh, add the mask feather filter.
Another way to do this, but more time intensive is to use the 4 point or 8 point garbage matte.