You tagged Elmah as your error reporting tool, so:
https://code.google.com/p/elmah/wiki/ErrorFiltering
To add an exception filter programmatically, put the following in your Global.asax file (or in the code-behind file):
// Don't forget to reference the Elmah assembly and import its namespace.
void ErrorLog_Filtering(object sender, ExceptionFilterEventArgs e)
{
if (e.Exception.GetBaseException() is HttpRequestValidationException)
e.Dismiss();
}