How to destroy/stop observable interval

you must have created a subscrtiption based on your this.countDown, you can destroy it in ngOnDestroy,

Excerpt from documentation,

Put cleanup logic in ngOnDestroy(), the logic that must run before Angular destroys the directive.

....
this.subscription = this.countDown.subscribe(...)
...

public ngOnDestroy() {
    this.subscription.unsubscribe();
}

You can use takeUntil operator.

import { Subject } from 'rxjs/Subject';
import 'rxjs/add/operator/takeUntil';

...

private onDestroy$ = new Subject<void>();

ngOnDestroy(): void {
    this.onDestroy$.next();
}

ngOnInit() {
    this.countDown = Observable.interval(1000)
                               .takeUntil(this.onDestroy$) // <----
                               .map(
...