If you have access via FTP to your website's Public html folder, you can ban this/these troublesome visitor/s from accessing your domain in your .htaccess file.
Open the .htaccess file and insert the following code beneath whatever may already be there. Add a (deny from) for each IP or domain instance you wish to ban as indicated. You can ban a specfic IP or an entire range of Ips by simply inserting 54. or 54.76. 0r 54.76.89. etc. You can ban all the users of a specific Internet service provider or website by simply denying their sites/servers web address.
#code below#
<Files 403.shtml>
order allow,deny
allow from all
</Files>
deny from 54.76.89.12
deny from 52.
deny from webhost.com
deny from subdomain.webhost.com
#code above#
The problem is, your perpetrator may have a dynamic IP or be using a public anonymising proxy such as 'hidemyass' or 'anonymouse' which will make his IP different each time he visits your site.
Are you using vBulletin? If so, here's my advice if you are worried about previous members with Admin. priviliges accessing the Administrator panel. Simply make sure you only have one primary administrator authorised in your config.php file. The ID will be that of the primary administrator's user name. 12,67,546 etc.
Open your forum's config.php found in your vBull includes folder.
Look for this. Make sure there are no ID's in those grey highlighted areas other than the ID's of users you want to have those permissions.:
As far as back doors? Of course there is not much you can do to change your database, but you can upload a fresh copy of vBulletin and simply indicate the old database name, user and password in the new config.php file to seamlessly move your forum to the new vBulletin installation.
Other than these things, simple moderation should protect your board from any mischief making. Just be sure to have html disabled in all your forum sections. When hackers want to disable a Forum with redirects, they usually try to insert malicious html or a booby trapped image. And remember that vBulletin hacks such as cybernetec's vShout and Advanced forum statistics are especially vulnerable to outside hacking.
Best of luck.