Hi Jorge,
It seems for displaying "USxxxxxx" in title line, you must be using &WF_INITIATOR& in work item text description, and WF_INITIATOR always carries US as prefix.
You need write some method to store &WF_INITIATOR& in some variable, separate out "US" from that variable, and get this new variable in workflow through bindings.
& Use this new variable in place of &WF_INITIATOR&. Now the user name will not have prefix US.
Regards,
Jimit