Do you mean an auto-renewing subscription ? If you do then there are instructions on this page for managing and stopping them : http://support.apple.com/kb/HT4098
If you mean iTunes Match then you can turn off auto-renewal : http://support.apple.com/kb/HT4914
If you want to cancel iTunes Match during it's current period then try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.
Or do you mean something else ?