No you can't. SSH isn't the only way to access the iPhone's file system. However, if you're not jailbroken, you do not have access to the root partition... only the mobile/Media folder. Changing SSH passwords or even file permissions won't fix that.
Can't what? If you restore the phone then all the data is erased anyway. If you are not jailbroken and somebody steals your phone, then they could easily jailbreak it to gain access to your data.