Well, I suppose it depends who they are, how easily they take offence and what the consequences are of offending them
You receive valuable messages from them and want to carry on like that but don't want other messages like abcxyz? So just reply back politely saying exactly that.
If it happens so often that you need a rule then you could set up a rule to automatically move their messages to a specific folder. Most emails apps will also let you run a rule on just the selected messages, so select the junk ones and then run the 'no thanks' rule on those.
Or if you're more like me, do a reply to all recipients, saying 'Hi all - I just got this bit of spam from poiuytrewq - I'm not interested but since you were all on the To:' list I guess you are; I thought I'd forward it again to you all just in case the original got inadvertently caught by your spam filters.'