Sometimes it requires a hard "no" to stop that player from ruining the rest of the group's fun.

I can often get away with "softer" nos, such as gatekeeping min-max powergaming templates behind tests that most powergamers can't pass. The most effective of those is "if a commoner, a barkeep, or maybe a wandering bard were to describe your character's calling and profession, in setting, what would they say about you?"

That often works because people that make templates to fulfill a character or story concept can easily pass it. The powergamer usually just sees stackable perks and nothing else with a "Kirito Stark" style cringe name stamped on top of it, with no family or regional associations.