Reset to factory defaults should do it. I just had a problem with mine that got resolved when Amazon support had me reset it to factory defaults; that process removed the Superuser app from my Fire HD...
Keep in mind that you will lose any data you have stored on your Fire; So you'll want to back up any pictures, docs, etc. before you do it.
Just slide your finger down from the top at the home screen to open the notification bar. From there select More, Device, then Reset To Factory Defaults. After a minute or 2, it will power back up and you will be prompted to set it up as if it were a brand new Fire. Select your language, enter your Amazon credentials, and follow the rest of the prompts to get it set back up.
I haven't decided if I am going to re-root, so for now I am using FrostWire (available from the Amazon App Store) to send apks from my phone and other tablet that I want, but can't get from the Play Store on the unrooted device. You can find tips online about how to use it, or I can try to give you instructions if you need them...
-sent from my Kindle Fire HD using Android Central the app.
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