Are these pages in a separate folder or under the root folder directly? Sitemap trimming only hides the links to the pages in the menu, it won't prohibit access to the secured pages. You have to apply some sort of restriction separately. You can do that by <location> tag in web.config file like this:
<location path="SecuredFolder/SecuredPage.aspx">
<system.web>
<authorization>
<allow roles="Administrators, Users"/>
<deny users="*"/>
</authorization>
</system.web>
</location>