How can I hide my calendar from the world?

I'd like my calendar to be private, so I have a password on the website where I have it embedded. However, I noticed that if someone entered the generic url for the calendar (i.e. http://spongecell.com/boxed_calendar/) and then guessed the 5-digit number (i.e. 56985) they could view my calendar. Is there any way to hide the calendar or password protect it? Thanks.