This post explains how to disable mod_security using .htaccess for one domain. You may have problem with mod_security if you are using certain plugins, wordpress, etc.
Go to your public_html directory of the desired domain and in .htaccess file add the following lines.
#vi .htaccess
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
Thats it. Mod_sec has been disabled now.
