I found it!

There's a quote that gets kicked around a lot: "You can't conquer a free man, the most you can do is kill him." I was pretty sure Heinlein said it - because between him and H. L. Mencken they've got all the good aphorisms tied up - and I sort of remember reading it. But for the life of me I could never remember where. And I was pretty sure there was more to it.

Finally found it this morning, quite by accident.

"I began to sense faintly that secrecy is the keystone of all tyranny.  Not force, but secrecy ... censorship.  When any government - or any church, for that matter - undertakes to say to its subjects, "This you may not read, this you may not see, this you are forbidden to know," the end result is tyranny and oppression, no matter how holy the motives. Mighty little force is needed to control a man whose mind has been hoodwinked; Contrariwise, no amount of force can control a free man, a man whose mind is free.  No, not the rack, not fission bombs, not anything - you can't conquer a free man; the most you can do is kill him."

- Robert A. Heinlein, "If This Goes On ---"