Could you please confirm that you handled the client-side OnDayRender event of the picker Calendar as shown in the code library sample?
Additionally, if you want to disable the desired days for other months server-side try setting the picker AutoPostBack property to true and see if it makes any difference.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.