I'm not even sure you CAN hide them; you have to be able to access the URL in order to invoke the service properly, and the .svc markups don't allow ASP content to be displayed instead of the basic metadata pages. I think the only way you could do it is with a service in front of IIS, evaluating requests and redirecting "naked" service requests to a catcher page.
Being able to browse to the page is a useful diagnostic and DIY tool. If you can see the page from the browser of the computer with the client software, then you should be able to connect to that service using the client software. So, unless you can think of a security or aesthetic reason why the page cannot be exposed in your production site, I'd just leave it be.