Thanks. I found an easier way -- deleting my PayPal account.
Of which the efficacy is certainly arguable, since a truly reputable firm might seek to provide such a mechanism in an easy-to-use manner. Whereas a shady operation would, and do, ignore the directive anyhow.
These days, messages in my inbox fall into two categories: Messages with an "unsubscribe" link, or spam.
This didn't use to happen in the olden days.