One option is to create a custom field that will remove the Assigned User if they are the same as the User creating the Activity, but ONLY for a specific Activity Type. 

This example assumes your custom Activity Type is named "RFC".

rfc_activity = self.env['mail.activity.type'].search([('name','=','RFC')])
for record in self:
    if not rfc_activity:
        record['x_clear_user'] = False
    else:
        if record.activity_type_id.id == rfc_activity[0].id and record.user_id.id == self.env.user.id:
          record['user_id'] = False
          record['x_clear_user'] = False

You then need to add this custom field to the UI, so create a Custom View like this that inherits the base mail.activity View and adds the field: