Sounds like a glitch. Have you tried restarting and/or reseting the device? You might also try Reset All Settings, though you'll be in for some tweaking afterwards if you do.
Another possibility is the
PhoneClean app on a computer. I've never used it, but others are claiming it works. While not for your specific problem, if the issue is lingering caches it might do the trick.
I'd recommend a solid backup before you try it, just in case.
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The usual fixes:
When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart. takes a bit longer than a restart.
Reset All Settings: Another, somewhat more drastic thing you can do is Reset All Settings. You'll find this under Settings > General > Reset > Reset All Settings. It won't erase any of your content, apps, or accounts; but you will have to go through most of your other settings and re-tweak them back to your liking.
That is unless it's a one time history erase, and you did a backup prior to the iMessage or content you are trying to remove and restored it back to that point. For most people this is NOT a good or viable option because you lose every app, document, data you had prior to the restore point and definitely everything if you start fresh with a new ID. 
