The legal authority part is far less of an issue. Governments have the power to control what does or does not cross their borders. Trump’s authority in that regard isn’t infinite, but he does have access to a number of legal mechanisms that could likely enable, at the very least, a temporary pause on trade. It would likely turn into another supreme court fight like the stuff with the tariffs. But, perversely, shutting off trade entirely with a country, rather than tariffing it, is actually more likely to fall within the framing of the IEEPA, for one thing. It grants the president the authority to “regulate” trade under certain conditions. It was ruled that this doesn’t include tariffs - that’s a power of Congress - but simply saying “Nothing crosses our border” might actually be a more clear cut case of “regulating” (ie, controlling).
The US doesn’t really trade with Spain. American citizens and corporations do.
On what legal authority does he think he can control what Americans do?
The legal authority part is far less of an issue. Governments have the power to control what does or does not cross their borders. Trump’s authority in that regard isn’t infinite, but he does have access to a number of legal mechanisms that could likely enable, at the very least, a temporary pause on trade. It would likely turn into another supreme court fight like the stuff with the tariffs. But, perversely, shutting off trade entirely with a country, rather than tariffing it, is actually more likely to fall within the framing of the IEEPA, for one thing. It grants the president the authority to “regulate” trade under certain conditions. It was ruled that this doesn’t include tariffs - that’s a power of Congress - but simply saying “Nothing crosses our border” might actually be a more clear cut case of “regulating” (ie, controlling).