First...remember that if you make this change, you will most likely need to redo the change after updates, as this is editing a core plugin file that could be altered with new versions of Blesta.
You will need to edit this file: plugins/order/views/templates/standard/types/registration/main_signup.pdt
Find:
<?php
$this->Form->fieldRadio("username_type", "email", ($this->Html->ifSet($vars->username_type, "email") == "email"), array("id"=>"username_type_email"));
$this->Form->label($this->_("Main.signup.field_username_type_email", true), "username_type_email", array("class"=>"inline"));
$this->Form->fieldRadio("username_type", "username", ($this->Html->ifSet($vars->username_type) == "username"), array("id"=>"username_type_username"));
$this->Form->label($this->_("Main.signup.field_username_type_username", true), "username_type_username", array("class"=>"inline"));
?>
Replace with:
<?php
$this->Form->fieldRadio("username_type", "email", ($this->Html->ifSet($vars->username_type, "email") == "email"), array("id"=>"username_type_email"));
$this->Form->label($this->_("Main.signup.field_username_type_email", true), "username_type_email", array("class"=>"inline"));
?>
This will simply remove the username radio button, so the only option is to choose email.