You don't want to turn on the microwave while they are in there because they will explode and make a mess and become a health hazard. What I suggest is using double sided sticky tape around the perimeter of the exterior vent (or the little holes). The wider the better. Their little feet will stick to the tape while both going in and out of the vent. Eventually they will figure out that it isn't something they want to do or there won't be any left because they will mostly have been trapped by the tape. You want them to die outside, not inside the home. Also, there is something that is attracting them to that area. Check for spillage of any sweet (or not so sweet) food items that may be attracting them. Sometimes it's just the fact that they can come in to get a drop of water and go back out so check for moisture problems as well.
Hope I Helped :)