There are several possible reasons why we cannot see of some user's ID with the naked eyes:
a) He/she, is a trained hacker!
b) Perhaps we have set the YM to hide any "Offline" users? Which means if some active users went "Invisible", they are consider offline too, & thus, their IDs nor Nickname will not appear in the lists.
Try these steps:1) Try clicking on " Contact " at the top of Yahoo! Messenger's list screen.
2) When it pop-down, move your cursor to the " Show/Hide ".
3) Make sure " Offline Contacts " & " Address Book Contacts" are selected.
If it works, & you wish to banned the user, follow advices from this link:
**P/s: Let us hope that she was not that advance, or else I have to provide with more delicately advance solutions =') . Good Luck!