web.config を使用して明示的なアクセスを制御できるはずです。この例を見てください (エクスクレーマー:この MS ページから直接コピーしました):
<configuration>
<system.web>
<authentication mode="Forms" >
<forms loginUrl="login.aspx" name=".ASPNETAUTH" protection="None" path="/" timeout="20" >
</forms>
</authentication>
<!-- This section denies access to all files in this application except for those that you have not explicitly specified by using another setting. -->
<authorization>
<deny users="?" />
</authorization>
</system.web>
<!-- This section gives the unauthenticated user access to the Default1.aspx page only. It is located in the same folder as this configuration file. -->
<location path="default1.aspx">
<system.web>
<authorization>
<allow users ="*" />
</authorization>
</system.web>
</location>
<!-- This section gives the unauthenticated user access to all of the files that are stored in the Subdir1 folder. -->
<location path="subdir1">
<system.web>
<authorization>
<allow users ="*" />
</authorization>
</system.web>
</location>
</configuration>
編集:明示的なフォルダーへのアクセスを拒否する方法の詳細については、この質問をご覧ください。