Ask HN: Is it possible to get an 18-year-old to spend less time on the computer?

A relative of mine asked for help with their 18-year-old child and computer time. The teen spends 9+ hours a day playing games and watching anime. When I was a kid, I got around a lot of restrictions, so I'm concerned a software-based approach won't work. The parent has tried locking the computer in a different room, which works, but it lacks nuance. It's okay to play video games as long as it is a healthy amount.

Parents of HN, what measures or completely different approaches have worked to establish healthy computer time limits for your kids?