If you are using HTTP BASIC authentication the only way to complete the logoff is closing the browser. The SSO cookie can be cleared but this must done the server side (that will tell the browser to discard the cookie). I'm able to clear the cookie on the ABAP WebAS but I don't know how to do that on Hana. I suggest you to contact SAP because this is a common requirement ant thy must provide a way to clear the SSO cookie.
Best regards,
Christian