Didn't we already go through this in the mid/late 1990s?
(I'd personally have a really hard time giving them a polite multi-page legal letter saying "sorry, we are unable to comply, and we don't have to, due to x, y, z" -- either a single "No." or perhaps "Nuts!", or trolling them with ASCII art or a return letter demanding NSA turn over their keys. Which is why I'm not a lawyer.)
(I'd personally have a really hard time giving them a polite multi-page legal letter saying "sorry, we are unable to comply, and we don't have to, due to x, y, z" -- either a single "No." or perhaps "Nuts!", or trolling them with ASCII art or a return letter demanding NSA turn over their keys. Which is why I'm not a lawyer.)