Sure they could. The ruling simply said that the Feds could not force a state government to enforce Federal law within it's boundaries. But Federal authorities could enforce it, anywhere within that state, just as as was done with the Fugitive Slave Act which was actively enforced by Federal officials and Federal courts.
The Constitution and laws made in pursuance their of are the only enforceable federal 'laws' outside the areas of exclusive jurisdiction.
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Please show me the 'law' that says a State, once it joined the Union, could not leave when notification was given.