You cannot disable it, but you can generate an arbitrary username placeholder for an arbitrary address or email address when a user logs in using an email address. Something like this in your reg form clean () method will make a unique username from an email address (since you cannot have usernames with nonalphanumerics):
highest_user_id = User.objects.all().order_by('-id')[0].id
leading_part_of_email = user_email.split('@',1)[0]
leading_part_of_email = re.sub(r'[^a-zA-Z0-9+]', '', leading_part_of_email)
truncated_part_of_email = leading_part_of_email[:3] + leading_part_of_email[-3:]
derived_username = '%s%s' % (truncated_part_of_email, highest_user_id+1)
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