We've never used it, and hopefully, we never will
You might have been watching TV before and heard an emergency alert interrupt your show with information on weather or news. A similar new text messaging system will soon be online to warn you of severe weather alerts (and a mysterious Presidential Alert). Turns out there are several levels of interruption including the most serious, a total takeover of the US airwaves by the President. MeFi member eriko explains it all:
Despite the fact that really the only message the President would send is "So this is it. We're all going to die," the capability remains, and by law, is required to remain. So, WEA may allow you to block advisories, like AMBER alerts (CAE -- Child Abduction Emergency, but sent with an Advisory/Statement flag) but you will not be able to block an EAN.