We've had enough of spam from a group of servers that is being a serious nuisance. And they all have one thing in common: they are investment related, in one form or another.
Working on the assumption that, if all victims adopt this policy spammers can be at least a little frustrated, this is what we have written to the company hosting the servers because, by reducing the business the ISP can do, there may be some pressure to be brought to bear on them to prevent their servers being used for spam.