Well, my first bit of advice would be to replace the stereo...
You can get a very decent CD/Mp3 playing deck for very cheap now. I think I picked one up at Wal Mart for $70.
If that's not an option, you can always dissect the stock stereo to remove the tape.
Tape players are fairly mechanically simple. I have ripped apart hundreds in my early years
There are guides online for how to remove the stereo in our cars, but it's very straight forward anyways...I'm sure you can figure it out. Take the -ve battery cable off first of course.
The internals of a cassette deck consist of the PCB's, and the tape deck itself is a metal cage with assemblies around it. If you open up the stereo you can most likely get at the tape and remove it with relative ease from the metal cage inside and then put it all back together. Good luck!