You are right, but on guest access portal solution you can block specific pages. It can be done using Access list filters. But yes it is little bit difficult. Please study the online help carefully. Rules are written using easy text field (like permit,ip source,ip destination, etc......). It is very simple and you cannot block web category or something like this.
For this client must be connected using Access controlled/authentication VSC. But there are some other ways how to do this (for example using firewall solution like Fortigate).
But HP is now intensively evolving and after Aruba acquisition we will have lot of options to block specific apps, webs, web category, built in directly to AP.