Well, if there is a postback on NodeClick event, then you probably are handling this event on the server. Simply remove the handler if it's not needed.
If you want to conditionally postback, this is another thing - you need to handle the ClientNodeClicking event and if you don't want to post-back, then args.set_cancel(true). If you want to still get the changes that this click does - add them in the handler of ClientNodeClick - for example:
node = args.get_node();
// for selecting the node even when the click is canceled
If this is still not what you are looking for, please, explain in a bit more detail what seems to be the problem and if needed, please, illustrate it with some code, too
the Telerik team