If a user in Outlook opens the shared folder dialogue, he should be asked if one of the users does not exist anymore:
As KOE tries to get a folderlist for these users, KOE should interpret the error returned from Z-Push to remove the store from the list. I would suggest to confirm that with a popup in KOE:
No data can be retrieved for user "deleted". Do you want to remove the share from the list: Y/N
It might be helpful to keep it around e.g. when you temporarily remove/unhook a store.