Put him into a medically induced coma, intravenously feed him liquified kittens, and force him to listen to Nickelback for all eternity.

His death is the punishment

The Electric Chair!

Public execution, if we’re talking historically

Bamboo torture until it kills him.

Have the smile demon haunt him forever but he can't kill himself

With an eternal fall

My personally i like him and i kinda support his goal.and ofc i will help him to achive his goal

Have him burn in the 9 circles

Use clamps to pull off his every single nail (sorry that's possibly the most sadistic punishment I can think up)