How can I cancel all my iTunes applications?

Cancel? How? If you have bought them you own them. If you mean a subscription (which is not the same as an "application")

View, change, or cancel your subscriptions - https://support.apple.com/HT202039 - "Learn how to view your subscriptions that renew automatically, change your subscription period, and turn off Automatic Renewal." "If you don’t cancel more than 24 hours before your renewal date, your subscription might renew."

You can then delete the app using the subscription but deleting an app alone does not cancel a subscription.