The full back plot is below but in summary to opt out of Paypal marketing you need to visit the link below and use your Ebay account. Note this probably only works if you are in the UK and I've found no way of doing it with my Paypal account - please comment below if you do. For why it needs an Ebay account read the next section below (I'm assuming you're impatient like me and just want to opt out without faffing).
Paypal's new policy (6 Marketing, Effective Date: Nov 01, 2010) states that to opt out of marketing:
simply indicate your preference by logging into your account and going to the Profile subtab under the My Account tab and adjusting your preferences under Account Information, or by following the directions provided with the communication or advertisement.If you go to this section on your account you have a number of check boxes such as "Partner/ Third Party Promotions" but don't be misled - there are NONE FOR MARKETING. If you click the link under "Marketing Preferences" instead you'll be taken to an information page and the last expandable section contains instructions to opt out by clicking an about or adchoice links above adverts themselves!
One problem... as far as I can tell Paypal do not have adverts of this nature... Ebay DO! If you're lucky(?) enough to have an Ebay account go to their home page without the Adblock plugin enabled on your web browser and click through from the tiny Adchoice link above any advert. Once you have done this you need to choose the option that is NOT "for this browser" (i.e. the quick, don't want to read, opt out option), instead you will need to log in using your Ebay credentials(!) to opt out of their marketing!!!! Feel free to follow the instructions above but for your convenience/speed I linked the (UK only?) "Adchoice" page directly above.
I can only assume that because Ebay ties your Ebay account with the Paypal account this option affects both accounts (Paypal+Ebay) marketing preferences. If this is not that case or you find out how to do Paypal's drop me a comment.