This is an important issue in some chat programs. I had to deal with this all the time: malicious users using i vs. l to pose as others, and using unicode to mess up or reverse the entire chat. One of the more interesting unicode had characters going left, right, and up and down. This confused moderators about who to kick/ban and obscured other users' text.

The solution was to implement a regex of whitelisted characters; since it's an English-only program, this works well and is future-safe. For multiple languages, a blacklist is probably okay, but the difficulty lies in keeping the blacklist both complete and up to date.