The web.config file controls all compilation settings in a default web site project. To treat compiler warnings as errors, you'll need the following in web.config:
<system.codedom>
<compilers>
<compiler
language="c#;cs;csharp" extension=".cs"
compilerOptions="/warnaserror"
type="Microsoft.CSharp.CSharpCodeProvider,
System, Version=2.0.0.0, Culture=neutral,
PublicKeyToken=b77a5c561934e089" />
</compilers>
</system.codedom>
Note: You can only twiddle with compiler settings under full trust, but then you'll only need this setting at build time.