Most people use word processors (not me!), and most modern word processors have some sort of auto-correct feature. What you can have fun with, is the fact that almost all of these auto-correct features are editable. Last time I used Microsoft Word I think it was Tools->Autocorrect Options, but I could be wrong.
Anyway, do whatever you want from there. I'm pretty sure it doesn't limit the length of the "correction", and it doesn't care if you're auto-correcting a dictionary word, so the next time someone types "I" or "the", you can have whole sentences popping out of nowhere, with the message of your choice. You could also make simple changes that would be frustrating before the victim realizes what's going on. Maybe 'the' to 'teh', turn "I" into "i" or add a simple typo in their name.