How do I cancel the doomsday subscription?

I believe the doomsday event bonus subscription is a short term item that runs out after a few weeks. If it is a subscription that renews you may be able to find details of any regular payments you have set up in your devices app store (google play store).

In google android this is found in the Play store by opening the left menu and selecting Subscriptions. Any recurring payments you have set up should be shown there.

There should be a similar option in iTunes.