Nicklist "dialog box"? The nicklist is a listview on the right of the channel. Just checking if we're talking about the same thing.
You can "auto-hide" the nicklist. This makes it disappear when the mouse is not over the right side of the window. It appears if you put your mouse over the right side of the window. Tell them to try this. If it pops up, they are using the auto-hide feature.
To disable it, click the window icon and go to Nick List... -- note that the setting is per-window. You need to select "set default" to apply the change to all windows.
