How can I get the iPhone to "forget" certain words?
It sounds like it used to be possible to manually edit the user dictionary, but unfortunately it isn't anymore.

Just a guess, but perhaps you can have it forget them the same way it learned them — when it suggests the word, try tapping on the small [x] next to the suggestion. If you do this a few times it may "forget" the word.



You can reset the dictionary entirely by going to Settings > General > Reset: