How to respond to “philosophical” questions that aim to undermine a position or derail a discussion?
Is this colleague really an academic? How on earth did he get a job in academia? How does he manage to retain it? He sounds like a teenage boy from your description of his style of arguing.

He's a troll, so best ignored. He also sounds like he's realised he's way out of his depth, and this is bluster to cover it up.

Formulate a single sentence that lets him know he's been heard, but that it was irrelevant, and that you've already moved on. Use it. Vary it, but keep it short, and always end with an invitation to the interrupted speaker to carry on where they left off. The troll should get the message that he's been sidelined, and troll elsewhere.

"I don't think we need to surrender to nihilism just yet, do we? [turn back to the interrupted speaker] Now, as you were saying ..."

Then just exclude him from discussions in the future. Take your fellow discussants to another room, away from the troll, if you have to.