It's up to the user whenever they upload their picture manually, use Gravatar or don't add an picture. If they changed their mind in the future, they simply go to Settings and choose their pick whenever go to system-generated pictures, use Gravatar, or upload manually.
For further actions, you may consider blocking this person and/or reporting abuse
It's up to the user whenever they upload their picture manually, use Gravatar or don't add an picture. If they changed their mind in the future, they simply go to Settings and choose their pick whenever go to system-generated pictures, use Gravatar, or upload manually.