首先添加一个按钮，点击它的时候执行清缓存和cookie操作，这里我重写了导航的返回按钮：

-(void)leftBarButtonClicked{
    _webView = nil;
    [self cleanCacheAndCookie];
    [self.navigationController popViewControllerAnimated:YES];
}

/**清除缓存和cookie*/
- (void)cleanCacheAndCookie{
    //清除cookies
    NSHTTPCookie *cookie;
    NSHTTPCookieStorage *storage = [NSHTTPCookieStorage sharedHTTPCookieStorage];
    for (cookie in [storage cookies]){
        [storage deleteCookie:cookie];
    }
    //清除UIWebView的缓存
    [[NSURLCache sharedURLCache] removeAllCachedResponses];
    NSURLCache * cache = [NSURLCache sharedURLCache];
    [cache removeAllCachedResponses];
    [cache setDiskCapacity:0];
    [cache setMemoryCapacity:0];
}