I know from reading Gearheads and seeing some of the earlier footage (back before Robot Wars came to the UK) that chains were allowed, but ended up being banned. As far as I understand it, it used to be that robots could be immobilized by being "pinned." Once that was disallowed, chains and nets were disallowed as well.

(I think I've got that right.)

No idea of whether that prohibition is still in place, though.