I once broke one off by bashing it with a screwdriver but it took a long time. I thought it would just shatter and crack but it didn't. Those things are more durable than they look. It would have been significantly faster with a cassette removal tool but, like you, I didn't have the tool at the time.
never had a problem mine came off with no problem I only removed it when it had a crack in one side from a rock flinging at it. I figured if it was on there when I got the bike it had to have a purpose
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Disks behind cassettes are usually all plastic and can be cut with a pair of tin snips as far as you can reach then scored the rest of the way with a utility knife working from the far side of the hub.
Some disks mounted behing freewheels had metal centers and the only way to get rid of them was to remove the freewheel.
Well after seeing that the plastic is 3x the strength of normal packaging plastic it took some time. I used that flat-edge screwdriver, drilled a nice big hole near the outer disk, shoved the driver in it and wobbled it every direction. Then it made a nice little line from the center to perimeter and came off after pulling some peices off. The bad thing is thats not the problem with my bike now. I think its the rear derailleur causin the lock-ups. Hm.