I've been getting increasingly irritated by the fact that additional words I've saved to the T9 dictionary (because I used them maybe once) appear as the default option when typing a text message. There's no option to delete individual words or to reset the dictionary via the handset menu.
However, if you use a Nokia S60 handset, I've now discovered that you can delete the predictive text dictionary, reseting it to the initial factory state.
1. Using a file browser (I use the free SExplorer) delete the directory C:\Predic (delete the entire directory, not just the .dat file within it).
2. Power off and back on your handset.
3. The T9 dictionary is back to the factory settings.