I have been hacked so whenever I go to my forum index page it redirects me to
http://airsoftwarbox.com/index.php. I know how it's happening; in the Forums & Posts section of the admin control panel is his link but whenever I click on the Forum & Posts section it redirects me to his site. So I can't access my site and when I try to delete his link it redirects me. What do I do?
Did he claim FTP access?
I recommend you backup all your webspace data and databases ASAP.
Does the redirect occur after the page has loaded or before anything loads?
He might either have redirected with .htaccess or PHP or HTML Meta Refresh.
To see if it's some sort of XSS from the database, try downloading a backup of your database, extract it if you have to, then open it in a code editor and see where it finds matches of that URL.
Check your MySQL database where MyBB connects to, for the URL. If it is there, remove it.
Instead of the attacker using a deface page he is using:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Your Page Title</title>
<meta http-equiv="REFRESH" content="0;url=http://www.the-domain-you-want-to-redirect-to.com"></HEAD>
<BODY>
Optional page text here.
</BODY>
</HTML>
So that when someone visits your domain URL they are automatically being directed to the desired site the attacker chooses.