If your running the latest version of MyBB it's very unlikely he is legitimately hacking you.
You should first change your hosting passwords as this is where I suspect the problem is coming from.
Then you should check all your privileged usergroups for additional members, he may have created other accounts before he left.
Finally, follow this tutorial to maximize the security of your forum: http://community.mybboard.net/thread-44977.html
In particular rename the admin directory and .htaccess password protect it.
It may also be an idea to replace all your files with fresh copies in case he had access to the files at any stage and placed some malicious code in there.
You should first change your hosting passwords as this is where I suspect the problem is coming from.
Then you should check all your privileged usergroups for additional members, he may have created other accounts before he left.
Finally, follow this tutorial to maximize the security of your forum: http://community.mybboard.net/thread-44977.html
In particular rename the admin directory and .htaccess password protect it.
It may also be an idea to replace all your files with fresh copies in case he had access to the files at any stage and placed some malicious code in there.
MyBB Support team member.
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