There isn't a contract for just using iTunes, but if you don't want to use it anymore just remove any personal info from your iTunes account and stop using it - accounts can't currently be deleted, and any items that you bought/downloaded with it will remain tied to that account.
If you mean that you've subscribed to iTunes Match then there are instructions on this page for cancelling auto-renewal of it : http://support.apple.com/kb/HT4914