When you are in "Messages" and the keyboard is showing on your screen do the following...
1. Tap the "Dictation" key. It looks like a microphone and is on the bottom left side of the keyboard.
2. In the floating menu that comes up tap "Settings". It looks like a gear.
3. Located "Smart Typing" and tap "Predictive Text". Next tap "Disable".
4. In this section you can also turn off things like "Auto-Capitalization" and "Punctuation".