方式一：

dispatch_cancelable_block.h:

//
//  dispatch_cancelable_block.h
//  TestProj
//
//  Created by Wesley on 2016/11/17.
//  Copyright © 2016年 Wesley. All rights reserved.
//

#ifndef dispatch_cancelable_block_h
#define dispatch_cancelable_block_h

typedef void(^dispatch_cancelable_block_t)(BOOL cancel);

static dispatch_cancelable_block_t dispatch_after_delay(CGFloat delay, dispatch_block_t block) {
    if (block == nil)
        return nil;
    
    // First we have to create a new dispatch_cancelable_block_t and we also need to copy the
    // block given (if you want more explanations about the __block storage type, read this:
    // https://developer.apple.com/library/ios/documentation/cocoa/conceptual/Blocks/Articles/bxVariables.html#//apple_ref/doc/uid/TP40007502-CH6-SW6
    __block dispatch_cancelable_block_t cancelableBlock = nil;
    __block dispatch_block_t originalBlock = [block copy];
    
    // This block will be executed in NOW() + delay
    dispatch_cancelable_block_t delayBlock = ^(BOOL cancel){
        if (cancel == NO && originalBlock)
            dispatch_async(dispatch_get_main_queue(), originalBlock);
        
        // We don't want to hold any objects in the memory
        originalBlock = nil;
        cancelableBlock = nil;
    };
    
    cancelableBlock = [delayBlock copy];
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delay * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
        // We are now in the future (NOW() + delay). It means the block hasn't been canceled so we can execute it
        if (cancelableBlock)
            cancelableBlock(NO);
    });
    
    return cancelableBlock;
}

static void cancel_block(dispatch_cancelable_block_t block) {
    if (block == nil)
        return;
    
    block(YES);
}

#endif /* dispatch_cancelable_block_h */

Usage:

#import "dispatch_cancelable_block.h"

dispatch_cancelable_block_t cblock = dispatch_after_delay(2.0, ^(void){
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        for (int i = 0; i < 10; ++i) {
            void (^ablock)(BOOL) = ^(BOOL b) {
                NSLog(@"BOOL = %d", b);
            };
            ablock(YES);
            sleep(1.0);
        }
    });
});

cancel_block(cblock);

方式二：

NSObject+Blocks.h

//
//  NSObject+Blocks.h
//  TestProj
//
//  Created by Wesley on 2016/11/17.
//  Copyright © 2016年 Wesley. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject (Blocks)

+ (id)performBlock:(void (^)(void))block afterDelay:(NSTimeInterval)delay;
+ (id)performBlock:(void (^)(id arg))block withObject:(id)anObject afterDelay:(NSTimeInterval)delay;
- (id)performBlock:(void (^)(void))block afterDelay:(NSTimeInterval)delay;
- (id)performBlock:(void (^)(id arg))block withObject:(id)anObject afterDelay:(NSTimeInterval)delay;

+ (void)cancelBlock:(id)block;

@end

NSObject+Blocks.m

//
//  NSObject+Blocks.m
//  TestProj
//
//  Created by Wesley on 2016/11/17.
//  Copyright © 2016年 Wesley. All rights reserved.
//

#import "NSObject+Blocks.h"
#import <dispatch/dispatch.h>

static inline dispatch_time_t dTimeDelay(NSTimeInterval time) {
    int64_t delta = (int64_t)(NSEC_PER_SEC * time);
    return dispatch_time(DISPATCH_TIME_NOW, delta);
}

@implementation NSObject (Blocks)

+ (id)performBlock:(void (^)(void))block afterDelay:(NSTimeInterval)delay {
    if (!block) return nil;
    
    __block BOOL cancelled = NO;
    
    void (^wrappingBlock)(BOOL) = ^(BOOL cancel) {
        if (cancel) {
            cancelled = YES;
            return;
        }
        if (!cancelled)block();
    };

    wrappingBlock = [wrappingBlock copy];
    
    dispatch_after(dTimeDelay(delay), dispatch_get_main_queue(), ^{  wrappingBlock(NO); });
    
    return wrappingBlock;
}

+ (id)performBlock:(void (^)(id arg))block withObject:(id)anObject afterDelay:(NSTimeInterval)delay {
    if (!block) return nil;
    
    __block BOOL cancelled = NO;
    
    void (^wrappingBlock)(BOOL, id) = ^(BOOL cancel, id arg) {
        if (cancel) {
            cancelled = YES;
            return;
        }
        if (!cancelled) block(arg);
    };
    
    wrappingBlock = [wrappingBlock copy];
    dispatch_after(dTimeDelay(delay), dispatch_get_main_queue(), ^{  wrappingBlock(NO, anObject); });
    
    return wrappingBlock;
}

- (id)performBlock:(void (^)(void))block afterDelay:(NSTimeInterval)delay {
    if (!block) return nil;
    
    __block BOOL cancelled = NO;
    
    void (^wrappingBlock)(BOOL) = ^(BOOL cancel) {
        if (cancel) {
            cancelled = YES;
            return;
        }
        if (!cancelled) block();
    };

    wrappingBlock = [wrappingBlock copy];

    dispatch_after(dTimeDelay(delay), dispatch_get_main_queue(), ^{  wrappingBlock(NO); });

    return wrappingBlock;
}

- (id)performBlock:(void (^)(id arg))block withObject:(id)anObject afterDelay:(NSTimeInterval)delay {
    if (!block) return nil;
    
    __block BOOL cancelled = NO;
    
    void (^wrappingBlock)(BOOL, id) = ^(BOOL cancel, id arg) {
        if (cancel) {
            cancelled = YES;
            return;
        }
        if (!cancelled) block(arg);
    };
    
    wrappingBlock = [wrappingBlock copy];
    
    dispatch_after(dTimeDelay(delay), dispatch_get_main_queue(), ^{  wrappingBlock(NO, anObject); });
    
    return wrappingBlock;
}

+ (void) cancelBlock:(id)block {
    if (!block) return;
    void (^aWrappingBlock)(BOOL) = (void(^)(BOOL))block;
    aWrappingBlock(YES);
}

@end

Usage:

#import "NSObject+Blocks.h"

typedef void(^theBlock)(void);

theBlock _theblock = [self performBlock:^(void){
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        for (int i = 0; i < 10; ++i) {
            NSLog(@"cblock %d", i);
            sleep(1.0);c
        }
    });
} afterDelay:2.0];

[ViewController cancelBlock:_theblock];