A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
Disinfection
You can uninstall this trojan with the Application Manager.
Additional Details
PbBLister is a trojan for Series 60 1st and 2nd Edition phones.
Once PbBLister is installed, it "black lists" contacts in the phonebook that match its criteria. The result is that the phone user will not see the blocked contacts attempting to call or send messages.
The trojan uses parts of commercial software to gain its functionality.