Hacksaw. A dremmel with a cutoff blade would work, too. You can also buy segmented, break-off bolts that you grab with a pair of pliers after tight and break off the excess at one of the notches. Not worth it to reset the toilet now, but maybe next time. If you cut the bolt before you put the nut on, you may not get the nut on! You can get it off later because the nut acts like a thread chase and straightens out the bent threads you made by cutting it off.