How do you TOTALLY delete messages on ipad

Hi, so I share an iPad 3 with my family (running IOS6). I used the messages app a while back to talk to an old friend and thought I had deleted the messages because I deleted the whole convo in the app. Turns out when you use the search function the messages come up. Is there a way to remove them without wiping the ipad? My brother uses it more than me and I don't want to delete everything, yet I don't want him reading messages