1)take the wheel off the bike, take the cassette off the wheel, slide the disc off, reassemble. Or 2)find whatever cutting tool is available (tin snips, scissors, vise grip , dremel tool) and cut, pull and tear until the SOB comes off.
Seriously the tools to remove a cassette (cassette removal tool, chain whip, and wrench) cost less than $20, and its a skill you should have. Take a look at PArk Tool's website.