The creep itself must be owned by one of the neutral players (black), who are also hostile against you. However, I strongly recommend you to trigger up the bounty system.
To make them drop items upon death, simply see the item itself and check the "Dropped When Carrier Dies". Then simply see the properties for the creep and add the item to the list. If you spawn the creep through triggers, just make another trigger which creates a new item and gives it to the target.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.