Try writing to the store's customer service department. Losing a customer is not something a store wants. Losing a customer and gaining a reputation for harassing customers with bogus shoplifting allegations is definitely not something a store wants. Losing a customer, gaining that sort of reputation and losing the patronage of someone who tries to stop shoplifters is, well, I think you get the picture.
I highly doubt that you can be arrested as an accessory/accomplice simply by refusing to physically restrain your sticky-fingered companion; barring some evidence of conspiracy, physical proximity to a criminal doesn't make you a criminal yourself.
So complaining to customer service politely but firmly may result in a lift of the ban— and maybe a small gift card too!

