In our implementation of the LMS, users are all signing up for their account via an Enrollment Key, either forced through the Portal Settings, or specific to them based on circumstances. Enrollment Keys provide an option to set the criteria for Username. We opted for it to match the user's email address. This works great. But if as an Admin, you use the Add User function, there is no rule applied to the Username field requiring it to follow any guidelines. There should be a Portal setting, just like there already is for the Enrollment Key, and just like there is for the Bulk User upload screen, that locks down what you intend to be a Username.