There are many resources you can find regarding a Joomla hacked site, either by googling it, or simply searching in this site.
Here are some links:
Make sure you read them carefully and take all actions that apply to secure your site.
Now, why you are constantly being hacked, even after updating Joomla:
If you go through and read on the above links, you will realize that security isn't only a case of 1 thing, but many things combined. So for example some common issues: Someone knows your credentials (FTP, Hosting, Joomla, Database), you are hosting many other sites under your account and one remains vulnerable, an extension (can be disabled) is still out of date and insecure, your hosting company is inadequate to provide secure hosting, your server software is out-of-date and so on, or your own computers are infected and hackers gather sensitive information from them and so on.
Of course, when a website has been already hacked, there is a full list of actions that must be taken, in order to clean it up and secure it. If you simply updated Joomla core, then most likely I could say that you still have a backdoor in your system. Likely the hackers have uploaded their malicious files and they still have control.
Since website security isn't a simple topic, I would recommend to hire someone with the required experience to release you from the pain.