IMO 18 isn't a kid or even really a teenager any more, here in Australia you can live fully independently the second you hit 18. Point being; if this person wants to play games 8 hours a day then that's their prerogative BUT as they're an adult the parents are now under no obligation to fund that lifestyle.

> The parent has tried locking the computer in a different room, which works, but it lacks nuance.

This makes me think that it must be their computer? If so then they can just change a password or two and boom - problem solved.