Want to turn my phone in to service and need to hide messages?

Sorry, no. Other than passcode protecting your phone, there is no way to do that. Why don't you just create a current backup, then: Settings>General>Reset>Erase All Content & Settings. When you get the phone back, just restore it from the backup you created.

Can't hide. Remember they know the guts of the phone inside and out far better than you so they can access all. Follow wjosten's wise words.