IPhone 5 won't let me delete message conversations?

My iphone 5 won't let me delete message conversations that are really long? I'm trying to delete them because they are taking up a lot of memory so when i delete them they just come back and my phone freezes.

You can solve this problem by simply following this steps
-connect your iphone 5 to pc and open itunes and make a full back up of your iphone and save that back up in your pc
-after backup, restore you iphone with itunes after successfully restoring make backup with that saved backup file
(it works with non jailbreak iphone)
make sure if your iphone is not jailbroken
hope if it works.

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