Siri, the iPhone 4S’s voice-command feature, is certainly amazing. But it could also be used to play tricks on an unsuspecting iPhone 4S owner.

The problem is with the way Siri is accessed. Rather conveniently – for both you and any pranksters who happen to be passing – it lets you send text and email messages while the iPhone 4S is locked, even on phones that require a passcode. You just hold down the Home button to activate Siri, compose a message, and send it.

That’s a nifty time-saver, but anyone can speak into an iPhone 4S and have Siri carry out commands, which means someone could pick up your iPhone 4S, hold down the Home button, and dictate an embarrassing message that looks like you sent it.

There’s a solution: it involves using a passcode lock (which you can specify in the Settings app under General Passcode Lock). Once you’ve set a lock, all you need to do is turn off Siri access on the Passcode Lock settings screen.

Set a passcode lock and turn off Siri access to secure your messages

Once you’ve put those measures in place, nothing happens when you hold down the Home button of a locked phone; you have to enter a passcode. You miss out on some convenience, but for some people, that’s worth the sacrifice if it stops a malicious rival from sending a fake email in which you resign from your job.