a) restrict access to the web interface (using the address range limitation of "/ip service" or the firewalling features)
b) disable the www service (/ip service).
Option b) doesn't hurt, you only cannot download the winbox.exe file from the router, if you would need it. With 2.9 the WinBox WILL be able to download the package plugins (unlike in 2.8, where they were downloaded over HTTP from that webserver).
You cannot change the web page itself.