Please follow the below-provided procedure in order to remove the WHMCS Billing & Support section in cPanel for an account.
You will want to move aside or delete the desired user's direcory from /var/cpanel/integration/dynamicui/ from the command line.
You can move aside the desired user's directory and its contents to /root/whmcs_dynamicui_backup by running the following command:
mkdir -v /root/whmcs_dynamicui_backup; mv -v /var/cpanel/integration/dynamicui/USER /root/whmcs_dynamicui_backup
replacing USER with the desired cPanel user.