Those clamps are designed for installation at the rack factory with a special crimping tool, for one time use...most times when the boots are replaced/serviced the rack is off the car or just replaced outright with boots already installed. The actual seals are inside and the boots are just a backup to keep major dirt, etc. outside. If the inner seals are shot, the rack might be worn as well, so if you're replacing the boots to deal with fluid loss, I'd consider a fresh rack instead. If the boots are in decent shape, they might be ok to leave in place, especially if you're having issues getting them off, etc.. If it were me and I was so far in that I was doing the inner joints, I'd just replace the rack. How many miles on the car?
There are any number of cutters that will do the job, from mini-bolt cutters to long handle dikes to rotary cutting discs. Same thing you'd use to cut heavy wire, basically. You could split them with a small sharp chisel, but one deep cut and you'd risk marking up the sealing surface. Same if you use a cutoff wheel, so be careful not matter what the method.
But again, the more room you have to work, the more tools you can pick from to work with, which is why I suggested dropping the rack a bit. If you use a cutoff wheel, it's better to cut the raised crimp so you stay off the sealing surface. Just cut thru so you can spring the clip open and then remove it.