From my perspective this feature should be disabled by default, but it`s not and it`s very annoying from my point of view. Fortunately it can be disabled easily. By disabling Windows phone suggest text you actually disable few more features.
Go to Settings and under System select Keyboard option (Slide 1.).
Tap on language you use in order to change functionalities (Slide 2.). In this case it is English.
By default all options will be checked (Slide 3).
Deselect Suggest text option and next few options will be deselected to (Slide 4.).
Now you have disabled Suggested text on writing SMS`s or on Windows Store.
I hope thet this tutorial was informative to you and thank you for visiting this website.