Go to forms editor, select profile form and then move the 'Change Username' from the left-hand side ('Form Bricks') to the right-hand side ('Available Bricks').
Hey there, thanks for the help. That takes care of the customer-facing profile form, not the admin-side one. I'm trying to make it impossible to change a username from the admin side. So, if you were to go to USERS > BROWSER USERS > EDIT USER, the username would just be a read-only field. Thank you again. If anyone has an idea, please let me know. Regards!
Unfortunately this is impossible to implement without core files modification. You may need to modify /amember/application/default/controllers/AdminUsersCont'roller.php and change Am_Form_Admin_User::init() function which defines form elements.