What if Trump decides to stop repaying government debt? Who will stop him?

I don't live in the US but the majority of my pension and investments are linked to the USA. I simply don't understand why he hasn't been stopped by the courts or Congress or the Sentate.

He is making millions of Americans poorer, anyone with a pension would have lost out this past week. Are his supports so poor they don't care? I don't understand the lack of apathy.

What if he decided this isn't enough and goes for the nuclear option such as stopping the interest and principle repayments for Treasuries? Could he do this? I cannot imagine what this would unleash.