Quoth Lace Neil Singer
Same here. My mantra in this occasion is "I don't work for free", and I stick to it.
Sam here. I've been in the grocery business over 20 years now and not once have I ever worked off the clock.
If I'm told not to stay past a certain time, then I simply clock out, gather my things and go. I have had a couple of occasions where I've had a store manager ask me to stay off the clock but would apply it to my hours for the following week . . .
Umm, how about HELL NO? I work, I get paid - end of story. See ya later, alligator.
If you don't want to be that blunt about it to your Sup/Manager, just simply tell him/her that you have already made plans for after your shift is over. If there's any grief over it, just reiterate that you have things YOU need to do and stick to that.
They'll eventually STFU and learn to leave you alone, hopefully.