he's got carriage bolts holding the front bumper on, and he has them loose enough that the square part of the shank has disengaged from the square hole, but the corrosion on the threads makes the nut tight enough that he has to keep the bolt from spinning somehow. i would try a wrench on the flats on the shank of the bolt if you can get one in there. other than that, try having somebody hold the bolt down into the bumper to keep the square engaged. last resort, cut them off. it seems to me that a 7/16 carriage bolt is ideal for that hole, but i can't find them anywhere. everybody only has either 3/8 or 1/2. really though, the bolts are probably 12mm. just keep that in mind if you cut them.
btw dreamer, if you would have spent half as much time typing your question as i did typing my answer, you have a much greater chance of getting some help