Pyboto: remove an ip based rule from a security group

I've been reading through the Pyboto documentation and whilst I know how to add an ip-based rule to a security group, I have not been able to find a method remove an ip-based rule. The method remove_rule on a security group object, doesn't update the security group on the ec2 instance - so I'm not sure that will help me either.

Has anyone accomplished this before?