The only answers are (1) massive retaliation, i.e. everyone connected with it is fired and can never work for EU again (2) a Constitutional amendment, forcing

The only answers are (1) massive retaliation, i.e. everyone connected with it is fired and can never work for EU again (2) a Constitutional amendment, forcing a supermajority referendum result for any such laws in the future. (1) unlikely. But imagine that the EC put out a proposal mandating a swastika underneath the EU flag, everywhere. Most likely those people would all be blacklisted permanently. So it's not inconceivable. (2) probably doesn't exist in the EU, but maybe it's in a bylaw somewhere.