Go to your account page here and sign in using the email and password that you use to access that account and then turn off ther auto-renewal.
http://home.mcafee.com/Default.aspx?culture=EN-GB (See 'My Account' in the top right corner).
Then contact Customer Service to get a refund:
To speak with a Customer Service representative, call 020 79490107. You may be asked for your registered email address and order number. Telephone charges may apply.
Estimated wait time: 10-30 minutes
Hours of Operation: 09.00-18.00 local, Monday through Friday
Alternatively contact them by online Chat here: http://service.mcafee.com/LocaleSelect.aspx?lc=2057&sg=CS&pt=1&st=CHAT
Chat online with a Customer Service representative for answers to questions such as billing and account related inquiries.
Estimated wait time: 2 minutes
Hours of Operation: 09.00-17.00 local, Monday through Friday
There is also an email option which I don't recommend as a) it is the slowest method, and b) account matters are best dealt with one-on-one.