To disable the Windows taskbar icon, close the PortableApps.com Platform. Browse to X:\PortableApps\PortableApps.com\Data and open the file PortableAppsMenu.ini in your favorite text editor (Notepad is fine). In the section [DisplayOptions] add the following line:
Then save and restart the PortableApps.com Platform.
Note that this may not work on Windows 8.1 due to a Windows bug but will work once Windows 8.1 Update 1 is applied.