at first please try to add just onlistcall="return false;". list should be never opened.
If that will not works - that means that it's an issue and that event is not cancelable.. so the only you could do - try to call hide in that case (slightly werd as will blink) or to patch component as shown at wiki.
Thanks for spending time on my post.
onlistcall="return false;" -- is not working
I am unable catch your suggestion in richfaces way. Could you please give me a link to complete documentation of richfaces.