How can I cancel this purchase

You must first cancel any subscription automatic renewal (if you simply delete an app this does not cancel subscriptions):

View, change, or cancel your subscriptions [subscription automatic renewal] - https://support.apple.com/HT202039 - "Your membership or subscription might renew if you don't cancel your membership or subscription more than 24 hours before your renewal date." At this stage the renewal is considered a purchase. For a new subscription the sale is probably effective from when you first agree to subscribe. Canceling a subscription does not automatically start a refund since the existing subscription will normally continue until the end of its term and then just not renew.

If you wish a refund: Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html), but some countries have different laws so check terms for your country at: http://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no. To request a refund:

Get help with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store - https://support.apple.com/HT204084

To request a refund or: Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days" (Does not work for subscriptions, use the contacts below.)

If a reporting a problem request is unsuccessful you can also try:

Other ways to contact iTunes Store Support:

https://www.apple.com/emea/support/itunes/contact.html

https://support.apple.com/country-selector/itunes

Read use details for "free" apps. Many have in-app purchases and/or free trials which automatically turn into paid subscriptions after the trial ends.