I had to do something like that last year. Mine was glued in place (don't know if yours is), and it was hard, thin plastic which I just carefully broke out with hammer taps, taking care to not let any big chunks fall down inside. If yours is rubbery, I would drill a couple of 3/4" holes in it close to the center and see if I could pry it out, or twist it out. What size is your cap? If it's large, say 7" or so, it may be attached to a toilet flange that's already installed and maybe you have to twist it to disengage it. I 'm not a plumber, but that's my 2 cents as no one else seems to be responding. Hopefully, more experienced folks will chime in here. Please report back on your success (or lack thereof). In any case I wouldn't worry about damaging the cap since you are going to toss it; you just don't want to do anything too violent that might damage what's underneath.