Please check previous post about validation with silverlight.
Enable and Disable Validation or you can say manually adding and remove binding for control.
In previous post i bound myName TextBox to Name property of Customer, But some case you do not want to validate that control and to do so you need to clear binding. ( Simplest way as per my thinking).
1. Remove Binding in Silverlight or Disable Validation
// Clear all errors from validation summary. ( Review previous post XAML)
// Set Text to clear ( As per your need )
myName.Text = String.Empty;
//This call will cleae binding for TextProperty( Take any dependency property)
2. Adding Binding in Silverlight or Enable Validation.
Binding binding = new Binding("Name");
binding.ValidatesOnExceptions = true;
binding.NotifyOnValidationError = true;
binding.Mode = BindingMode.TwoWay;
Let me know your comment on this.