In the airline I recently retired from there were always handcuffs and law-enforcement tie-wraps onboard to restrain unruly passengers that became a threat to crew/passengers and/or flight safety. Captains authority. This is much safer than duct tape - especially in case of an emergency and the passenger has to be released for f.e. an evacuation. Duct tape might restrain a person, but is not safe should there be complications afterwards.