Hmmmm, if theyve added an alias, just put THAT in the YahooID box, put the MainIDs password (all aliases share the password of the parent ID) into the password box and then hit 'Connect' - childIDs can be logged in exactly the same as a ParentID.
Once youve done this you can reject people (remove the currently logged in ID from others lists).
The recent change by Yahoo was to remove the html method of contact rejection - JAM uses YMSG11.
Just as a side note - theres a link to DaB on my site(s)
http://www.geocities.com/troling_around
-or-
http://trolling_around.tripod.com
DaB (DenyABuddy) is a small utility that does as its name implies - it logs your ID in using web-messenger (which means your ID will get logged out of Yahoo if you use this whilst it is logged into messenger) and remove you name from someones list. (WARNING - DaB doesnt appear to work for all yahooID/rejectID combinations - tiz a yahoo thing
JAM is much more reliable (but a bigger download).