How do I render a lost iPhone useless to the person that has it?

iOS 7's Activation Lock, which is implemented as soon as you set up find my iPhone, will keep them from activating it and using it as their own. However, if there was no passcode lock in effect with the lock screen, then there is nothing keeping anyone from accessing the content of the device, or using it as it is.

If it is or comes back online, from your online iCloud account, you can erase it, which will make it untrackable but will also remove all content from it. The person would then be stuck as Activation Lock would keep them from resetting it and using it.

You can also contact your carrier and see if they can blacklist or block the IMEI number.

If you have Touch ID activated you also have a passcode to unlock the phone. The thief will not be able to get into it unless your passcode is very obvious like "1111". After a few tries the phone will block further attempts for longer and longer times. Also, if you had set the option to erase after 10 failed attempts that will completely defeat the thief. So essentially the phone is useless, but there's no way they can see your message unless the message was your lock screen wallpaper.

Without your Apple ID and password as well as the passcode, it will be useless to anyone that finds it.

http://support.apple.com/kb/HT5818