You need to modify the control template of RadMenuItem style in order to remove the MouseOver effect of RadMenuItems. Look for VisualState named Highlighted and remove its content. You can find all styles and resources of RadMenu inside installation folder on your machine. Inside that folder you will find the Theme.Implicit folder which contains all themes resources. Just drill down to the Telerik.Windows.Controls.Navigation.xaml file and theme of your choice.
I hope this would be helpful.