Although this may be inconvenient this happens sometimes. Not by a large, credit card company like Amex but by smaller business partners.
IMHO the only course of action is to approach the sender in a friendly and not humiliating way. He/She already will know by tons of negative comments that he/she made a fault.
And I see no reason for squeezing out a compensation in form of MR points if you already want to end transactions with them. For me it seems a bit like being a freeloader.
You haven't had any harm and just would like to get a compensation for a minor annoyance. Due to the fact that I treat all business partners with respect squeezing out a compensation for nothing is far beyond all thinkable alternatives.
So no course of action IMHO!
I'm far from being an 'ambulance chaser', however it strikes me as crazy that anyone is able to do that. I do not blame the person that did it really, its the fact Amex allow it to happen. Especially when asking people to pay £450 a year, to become a cardholder! I've had some experience in 'email marketing' and know that this kind of information + the background of the people on the list (probably all Amex plat or cent, looked to get tickets for a certain event etc) would be worth a bit of money to sell this information on.
If I don't kick up a fuss, how will things like this ever get fixed!