If the disc is attached to the spokes, tin snips work. Rotate it so that the clips disengage from the spokes and snip it in a couple of places. Then break it away in sections. Also you can use a pair of garden shears. However, I did not receive a compliment from my wife for being resourceful the last time I did that. If it's mounted to the hub then that's a bit trickier and will likely require you to remove the cassette.

Also, assuming it's attached via clips to the spokes, another method is to ride around with it for a little bit and take a few potholes, hop some curbs, take it offroad... Pretty soon the thing will break apart on its own. Of course this is rather unfriendly to the environment so be sure to pick up the resulting pieces of plastic. Also, be aware that this may jeapordise your spokes should some of the bigger pieces get lodged against a spoke and seat/chainstay. And of course you also run the risk of those pieces getting into your derailleur and drivetrain...

But hey... it's a spoke protector afterall and is meant to protect things... right?