In order to enhance the functionality of the RadMenu control, the RadMenuItem can behave like a separator. In order to make it a separator you have to set its IsSeparator property to either True or False. The default value is False.
Here is a sample definition of RadMenuItem, used as separator:
<telerik:RadMenuItem IsSeparator="True" />
In order to customize the appearance of the separator you have to set the SeparatorTemplateKey to an appropriate ControlTemplate. To learn more about the RadMenuItem's templates read this topic.