All I can suggest is that you Log on with your usual username and password and then go to My Account which you will find as an option along the top of the page just under the headline Racing Post.
Here they hold all your Account information including any Credit Card numbers that you have given for your Membership.
There is also an option to view your Members Club Account/Wallet/Preferences etc but as I don't subscribe (and personally never will) to Members Club there is no info in that section for me.
If you click on the Members Club option you should then be given a choice to unsubscribe, if not then I have copied for you from the site a list of contact numbers:-
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COPIED FROM RACING POST SITE (Customer Service Info):-
If you have any website related queries, please e-mail the Racing Post Customer Services using one of the e-mail addresses below.
We will attempt to answer all emails within 24 hours of receiving them, during Customer Services hours.
The Racing Post Customer Services is manned from 8am until 10pm Monday to Saturday and 10am-8pm on Sundays.
Please direct your e-mails to the relevant address:
For internet queries: help@racingpost.com
For online shop queries: shop@racingpost.com
For advertising queries: advertising@racingpost.com
Other useful contact details:
Customer Services - +44 (0)1635 898 781 (8am-10pm Monday-Saturday and 10am-8pm Sundays and Bank Holidays).
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Good luck in cancelling your Members Club subscription, I can't believe that Racing Post ever started such a scheme when At The Races give all the information and more for FREE.
If you don't know already (and you probably do) the address is
http://attheraces.com
A good move IMO thats £90 that RP won't be seeing of your money this year, put it on a few horses of your own chosing and be lucky....Jac x