I sometimes have luck keeping this from happening by erasing the automatic dimension and manually making a new one, then "disassociating" the dimensions points by manually moving the grips somewhere away from the point I selected. Kind of a "place, move, move, replace" thing with each grip. Sometimes it works, sometimes it doesn't. If it doesn't I just let the dimension do its thing and try to ignore it because AS likes to have a mind of its own with parametric dims.