Is there any way to disable the Watchaguard VPN portal?
You can disable the SSLVPN page via the firebox's CLI. See:
(Plan Your Mobile VPN with SSL Configuration)https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/mvpn/ssl/configure_fb_for_mpvpn_ssl_c_before.html#Name
(instructions are in the section labeled "Software Downloads Page Hosted by the Firebox" near the bottom of the article.)
WatchGuard Customer Support