Aye, I see what you mean, so you can get an open end spanner on it. but it would have to be perfectly straight so the spanner don't slip. Too much fafin about I reckon.
Another method I use, is to cut through the head of the bolt (about two thirds of the depth) with a hacksaw, then hamer on a slotted screw fitting that then fits onto my impact driver, then give it a good wack to loosen.
Give the screw a quick squirt of WD40 and leave for half an hour, then go back to it. (Yeah I know, WD40 around brake disks. I clean all parts with paint thinners before refitting)
If you brake the rubber then order a new part with the bolts