Hey Janni,
Have you tried using the remove method that is displayed in this demo?
Add, remove, disable menu items
I'm not terribly good with all the in's and out's of RadMenu yet, but in my thinking .clear would do just that and clear what's there, but .remove would actually get rid of the child item... right?
If I were better with javascript I'd try some sample code, but I'm not, so if anyone knows how to implement this and wants to share, now would be a good time. >.<