Please look at this sample:
http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#EditToolsAutoSave
CancelActive is the command you use to cancel the current active command.
Clicking the "Clear Action" button with Command Binding to Editor's CancelActive is equivalent to this in code-behind:
if(editor.CancelActive.CanExecute(null))
editor.CancelActive.Execute(null);
where editor is your Editor instance, null is the CommandParameter.