code: def revoke_dbsecurity_group(self, group_name, ec2_security_group_name=None,
                                  ec2_security_group_owner_id=None, cidr_ip=None):
          
          params = {'DBSecurityGroupName':group_name}
          if ec2_security_group_name:
              params['EC2SecurityGroupName'] = ec2_security_group_name
          if ec2_security_group_owner_id:
              params['EC2SecurityGroupOwnerId'] = ec2_security_group_owner_id
          if cidr_ip:
              params['CIDRIP'] = cidr_ip
          return self.get_object('RevokeDBSecurityGroupIngress', params,
                                 DBSecurityGroup)
docs: Remove an existing rule from an existing security group.
          You need to pass in either ec2_security_group_name and
          ec2_security_group_owner_id OR a CIDR block.
  
          :type group_name: string
          :param group_name: The name of the security group you are removing
                             the rule from.
  
          :type ec2_security_group_name: string
          :param ec2_security_group_name: The name of the EC2 security group
                                          from which you are removing access.
  
          :type ec2_security_group_owner_id: string
          :param ec2_security_group_owner_id: The ID of the owner of the EC2
                                              security from which you are
                                              removing access.
  
          :type cidr_ip: string
          :param cidr_ip: The CIDR block from which you are removing access.
                          See http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing
  
          :rtype: bool
          :return: True if successful.