[Black Tiger]

You could also do a combination of things.
Backup your database for if you change your mind later, then all is easy to put back up.
Remove all files and place an index.html like Boofo said.
If you had your forums in a /forums directory, put the same index.html there to or use a htaccess redirect to the main index.html file.

I presume you already notified your users about your plans.