Well, if you got the right hardware/software to delete apps on iPad, you can erase just about anything from any computer/device. The only surefire way to prevent such is to toss your phone into a blast furnace. However, the right equipment to delete data from phone is not so easy, and generally not available to most users.
If you go to Settings>General>Reset>Erase all content and settings
Once you do that it will put the phone into setup mode with iPhone Data Eraser and everything will be deleted from the phone and they won't be able to get your information back.