Since there is no built-in Kill Switch (as of today) option in the SoftEther, I have been desperately looking all over the internet to find a working solution. No results so far.
Then I noticed, once you connect and disable the DNS configuration, make it a static 0.0.0.0, once you disconnect or drop from SoftEther, you simply cannot connect to the web. If you reconnect back the SoftEther Server you can again browse the web.
Am I mistaken or is this the same as what cedar meant, when he wrote: "Simply, you should remove the default gateway for ISP." in connection to the same topic: