Disable nicknames/display names

i am running a community based site, and users can comment on stories that are posted.

recently, they’ve figured out that they create different nicknames and have those nicknames be displayed when they add a comment, instead of their user name.

this wouldn’t be a problem except one or two of them are using nicknames that are VERY similar to other people’s user names, in an attempt to confuse folks, i think. now, i could just ban them for doing it, but i wondered if there wasn’t a way to just force WP to either display their username or disable nicknames altogether?

thanks!