Manual:
"Email notifications can be enabled or disabled by forums admin. Usually, on large forums they are disabled. If enabled, every user who makes a post, can subscribe to them."

Is there a way to setup the notifications for users as default and not cancelable?

Do you want your users about imposing a subscription? The decision it would be better to meet each user himself. Each user should decide for himself whether he wants a subscription, and when he again canceled. Or do you want to scare your users? ;-)

Thanks, maybe you are right.