Eurofed
Banned
Stick it in the 14th amendment.
Yes. A PoD might add a provision in the 14th Amendment that empowers the Congress to forbid and punish public and overt shows of allegiance to foreign enemies, rebellion, and insurrection. The courts would surely construct this to affirm statutes that forbid the public display of CSA flags.
A possible PoD: the Booth associate that was charged to kill Johnson succeeds. The twin assassination of POTUS and VPOTUS angers the North in supporting a harsher and longer Reconstruction, with more decisive military and legislative suppression of Southern revanchism and segregationism, and a broader and harsher 14th Amendment (which in addition to harsher sanctions agianst shows of support for enemies of the US gives stronger federal protection of civil rights against states' encroachment). While this has the dark side that sectionalism animosity and antagonism would linger longer, and an exception to the First Amendment would be enshrined in the Constitution (which would make the life of antiwar protesters more difficult in the 20th century: waving the flags of North Vietnam or the Vietcong during the Vietnam War would be a felony, and it is quite possible that the courts could construct flag-burning itself not to be protected by the 1st), it is the bright side that racial segregation would be butterflied away or greately diminished, and a stronger, broader 14th could be easily construed to protect all kinds of civil rights that IOTL have not been so or only later.