Joomla Website Files Spamming Investigation / Security

Hi, I have a client's site running the latest version of Joomla with Virtuemart hosted on a cpanel setup in WHM. Everything is running smoothly except the site keeps getting hacked by spammers who are using the email system, this is using up the resources and causing the server to get black listed. This originally started happening a few years ago using an old unsupported template and with an increasingly outdated version of Joomla. So last year the site was rebuilt and has been kept updated since, which solved the problem and the hacking stopped for nearly a year. Unfortunately it has started happening again over the last couple of months despite the software being up to date. I'm not sure how it is getting hacked. It's possible the site was vulnerable for a short period of time between updates or there is a plugin/module/component that is not up to date that I am unaware of. Or another reason entirely. I recently removed a B2J plugin when realising that it had become a premium plugin and was no longer getting updates, it's possible that that plugin has been the weak link and used as a way in for the spammer scripts. A lot of time has been spent scanning and cleaning up affected files over the years and what I would like now is for somebody with a lot of Joomla technical experience to investigate and hopefully find out why the site is getting hacked and stop it from happening. I have a list of files from a recent scan that have potentially malicious code injected in them, many of these have been deleted or cleaned up but several remain which may need checking. I look forward to hearing from you. Thanks! Mark