For Amex, I found this: https://global.americanexpress.com/account-management/privac... (Maybe the checkbox at the bottom does a similar thing)

Sadly that option doesn't even exist on the Canadian site for whatever reason.

I did manage to find this (https://optout.aboutads.info/?c=3&lang=en) which they claim to honour.

I probably should be purging cookies more often :/