Use a captured variable to allow a block to be cancelled in the midst of a long-running process. This is essentially the same as the NSOperation isCancelled flag.

cancellable_block.m

    __block BOOL isCancelled = NO;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        for (int i = 0; i < 10; i++) {
            NSLog(@"Loop %i", i);
            if (isCancelled) {
                break;
            } else {
                sleep(1);
            }
        }
    });
    
    sleep(3);
    isCancelled = YES;