If you are in chat rooms and you have selected,
"Ignore anyone who is not on my messenger list",
It will look like no one is talking. Everyone will be ignored if there not on your list!!!
If you ignore someone in a chat room they are ignored untill you turn off Yahoo messenger, No limitations on how many you ignore.
but if you ignore them from a PM box it gets added to your ignore list and that does have limitations, and it will remember next time you restart Yahoo.