Deleting iMessages on iPad
Without seeing a screenshot of what, exactly, you're talking about, it's hard to give any really good advice.

That said, try this:


Go to Settings
Scroll down to Messages; set iMessage to OFF
Back in Messages, go to the root screen (where you see all the different conversations listed); either click Edit and then swipe each conversation to delete as needed


This should get rid of all your messages, as well as prevent any further iMessages from being delivered to that device.