How do I turn off viewstate for good?

Turn off the ViewState by default by using a <page> element in the web.config. Using EnableViewState="true" in the @Page directive will no longer work once you disable the ViewState in the web.config. If you decide later that you need the ViewState for a specific page, you can turn it back on for just that page using a <location> element.

<configuration>
  <system.web>
    <pages enableViewState="false" />
  </system.web>

  <location path="MyFolder/MyPage.aspx">
    <system.web>
      <pages enableViewState="true" />
    </system.web>
  </location>
  <location path="Site.master">
    <system.web>
      <pages enableViewState="true" />
    </system.web>
  </location>
</configuration>

You need to do the same for any master pages that your ViewState enabled page uses.