Edit the theme file at: forums > front > index > index and delete:
{template="pageHeader" group="global" app="core" params="IPSMember::loggedIn()->language()->addToStack('forums')"}
or
Add the following to your Custom.css:
body[data-pageapp=forums][data-pagelocation=front][data-pagemodule=forums][data-pagecontroller=index] .ipsPageHeader { display:none; }
Second way is the recommended way.