To force clients to use their email address as their username you can do it either one of these ways:
If you are using the Magic Box you will need to do the following:
Add Custom css: simple
pages : client side
Markup to set in : head tag
CSS To Add :
#username_type_username {display:none}
[for="username_type_username"] { display: none;}
Please note this has been improved in Blesta 3.4 as CORE-547, where there will be an option to disable it in the settings.