Is it illegal for the organisers of a memorabilia fair to clamp cars so that traders cannot leave?

My fianc�'s business partner is currently at a toy fair at Birmingham's NEC, which was meant to finish at 4pm. Because some celebrity guest who's meant to be doing a signing is late, the organisers have decided to close at 5, and are claiming nobody's allowed to close their stalls until then. He's heard from other traders that the organisers are threatening to clamp the traders' cars to stop them leaving early. That can't be legal, can it?