https://discussions.apple.com/thread/4416769?start=0&tstart=0
Here's the solution to your problem:
EVERYONE on this discussion board (especially Meg St._Clair) should send the following message to Apple so that they can recognize how serious it is and fix this problem FAST with the NEXT UPDATE:
TO: http://www.apple.com/feedback/iphone.html (Apple.com Product Feedback)
RE: iPhone 5 Text Message Auto-Populate List or Pop-Up Short List (AutoComplete List) can NOT be Deleted
Apple, please fix this problem. Here’s the problem: When creating a text message, as you type the name/phone on the “To Line” of the person you are sending the text to, the iphone automatically pops-up a short list to choose from (similar to “AutoComplete” in Internet Explorer) of previous names that you have ever PREVIOUSLY sent a text message to. This short list is a DIFFERENT list from your “Phone’s Contact Address Book List”. This Auto-Populate feature stores these names and number in a different place on your iphone and even if you have DELETED the person’s name from your “iphone’s Contact Address Book List” that name still can NEVER be deleted from this “Auto-Populate” pop-up short list. The ONLY way to remove this data from your iphone is to restore your iphone as a new device in iTunes, NOT from the backup. However, you will LOSE ALL your contacts and information performing this. This is not a good fix because after a short while, the SAME problem will happen again. The fact that DELETED CONTACTS are auto populating and popping up as a short list in both emails and text messages (and can NOT be DELETED) is something that should have been fixed a long time ago! Please fix this problem with the next update. Thanks
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