The fact that 'items' is undefined implies that you never appropriately selected the tabstrip in the first place. Either your CSS selector is wrong (are you sure you named it tabstrip?) or you did not follow the Kendo method names appropriately.
Here are two ways I found to hide the last tab:
Hiding the last tabstrip element
var tabStrip = $("#tabstrip").kendoTabStrip().data("kendoTabStrip");
//Find the last tab item's index from the items list
var lastIndex = tabStrip.items().length - 1;
//Use jQuery's hide method on the element
$(tabStrip.items()[lastIndex]).hide();
Using Kendo's tabstrip remove method
I believe the following is more appropriate. Why not use the tabstrip's remove method and completely remove it from the DOM since the user should not have access anyway?
var tabStrip = $("#tabstrip").kendoTabStrip().data("kendoTabStrip");
tabStrip.remove("li:last");