The Supra C500 is a pushbutton combination lockbox... so Realtors would not have access to this lockbox unless someone gave them the combination.
Call the agent you used to buy the property, and ask him/her to contact the prior listing agent, and get them to remove the lockbox. The likelihood is that they just forgot that it's there. If, for some reason, you can't get any results through your agent... or by contacting the agent who originally listed your home... I would follow Scott's advice and get a set of bolt cutters and cut it off and discard it.