Can I turn off the alternate keyboards in Messages
Solution 1:
Assuming you're talking about the Handwriting Input panel in iOS 10:
Rotate the phone to show the input area, then tap the keyboard icon, showing the keyboard, and it will keep the setting.
Image / info via: https://www.gottabemobile.com/how-to-turn-off-the-5-most-annoying-ios-10-features/