The 39-year-old had Spinks down in the first round of the fight as he looked for an early finish, with the former champion backing into a defensive mode for much of the rest of the contest as Bundrage looked for the one-punch knockout.
The fight was marred with prolonged holding throughout, and referee Corona looked lost at how to separate the two fighters as they bumped heads and stayed inside for long spells in the fight.
The ending came with a huge right hand early in the seventh round that saw Spinks fall heavily to the canvas, and by rights, the fight should have been stopped there and then with the challenger out on his feet.
Corona allowed Spinks to continue not once but twice as he sprawled around the ring for the next two minutes hitting the floor intermittently, and can claim to be the victim of two low blows that the referee missed as Bundrage focused on the finish.
Towards the end of the seventh, Spinks coward in the corner as Bundrage jumped all over him, literally, and Corona had eventually seen enough, belatedly waving the fight off to the delight of the champion.
Bundrage could now be in with a chance of unifying the titles against WBC title-holder Saul Alvarez in the fall with a decision due next week on who the Mexican superstar will face on September 15.