By making it illegal and punishable with a long prison term. Something like "if you train a model more capable that what we have nowadays, you have to destroy it and notify the authorities within few hours, otherwise you go to prison and your company is dissolved".

"Smaller and more efficient" models are fine. It's "smarter" the problem.

Training new frontier models will likely require a huge amount of computational resources for a long time. Few companies worldwide are capable of that. It's not like someone will train a new GPT 6 - like model in their garage.