How to permanently delete an SMS or imeasage coversation without resetting the device everytime?

HI, I guess people might have noticed when you delete a conversation from the messaging app, text messages or imessages, they still appear on the spotlight. I have tried rebooting, syncing after deleting, nothing worked. the only option would be remove messages from spotlight search or reset the whole device..!!! Can anyone help me coz sometime unfortunately the messages have sensitive financial info and other personal messages that i woudnt like sneaky co-workers see. Thanks, Hari