Yrs this is years old, but maybe some Googler will find this helpful.. You absolutely cannot do this with SwiftKey. You can long press a prediction to tell SwiftKey to never predict that word again. But as soon as you type that word again, SwiftKey will add it back to the allowed predictions, rendering this feature all but useless. It's overwhelmingly frustrating, and I can't believe the developers refuse to address this much wanted feature