the ones without the allen key I will usually take the blade disc off and stand on the plate while undoing the bolts on a solid surface such as cement, to firse the bolt to grip on the little bit if the diamond shape hole that is left.
Another (albeit backyard) method I have used is to put the tinyest tackweld on the bolt to the plate, undo the nut then grind it down smooth again to remove the bolt.
