Or say "I'm not comfortable giving up my materials but you're welcome to shop with me." I've said this a few times as I'm walking off. For some reason security guards especially don't like to leave a five foot radius spot they're planted too. Store employees really don't seem to care.

Again -- If you keep walking, and respectfully with eye contact say NO, THANK YOU, My experience has been they leave me alone, or mumble something about, "just this once"...

I tend to give them as much thought as a guy asking me for money on the street. I'm not rude to anyone, but I don't have to stop and chat with everyone about life and policies, I just do my thing.

Agree with above, you're backpack was not impounded, you gave it to them. I'd shop somewhere else.