On the one hand there is [Userscript] Wanikani Procrastination Annihilation - #19 by Mempo which will hide the forum only when you have reviews to do.
And for hiding it permanently there is this custom style: Wanikani: Remove Useless Panels | Userstyles.org and an explanation for how to hide only the forum: Hiding the Forum - #6 by Kumirei