If a state seceded, the US government would not recognize it. The fed could simply declare the state to be in rebellion, and,
For starters, the US government could abolish every federal government job in that state,
cancel every government contract going to businesses in that state, close every federal building and installation,
including all military bases in that state,
and then suspend all federal government payments to residents of that state,
not the least of which would be Social Security and Medicare.