Brad Wilson describes this in his blog post: http://bradwilson.typepad.com/blog/2010/10/mvc3-unobtrusive-validation.html - I have highlighted the bits that answers your question (the last line) in the following quote from the blog post:
To turn unobtrusive JavaScript mode on/off and enable/disable client validation by default for the entire application, you can use Web.config:
<configuration>
<appSettings>
<add key="ClientValidationEnabled" value="true"/>
<add key="UnobtrusiveJavaScriptEnabled" value="true"/>
</appSettings>
</configuration>
You can also turn them on or off with code:
HtmlHelper.ClientValidationEnabled = true;
HtmlHelper.UnobtrusiveJavaScriptEnabled = true;
Using code to turn these features on or off actually behaves
contextually. If those lines of code are present in your Global.asax
file, then it turns unobtrusive JavaScript and client validation on or
off for the whole application. If they appear within your
controller or view, on the other hand, it will turn the features on or
off for the current action only.