Turn-off "Who's online" for Moderators

Hi, there

What I like to do is let only administrators see the "who's online" and turn off this feature for moderators and normal members. I now know how to do it, control panel -- user groups and permissions -- edit.

However, everytime I change the setting to be "no" to "Can View Who's Online" and save, it says "Record updated!" but when I click "edit" again to see whether it's really changed, it stays with "Yes" and it drives me nuts!

I tried thousand times but it simply not work. How can I turn this off for other members?

Thanks in advance.