If for TRS 2004, as long as you know the Kuid number of the loco to delete (in Surveyor click pointer on the loco while having the leftmost bottom icon on in the loco tag) and just remove its folder in TRS/world/dispatcher/downloads.
Or use TrainzObjectz (from Tafweb's web site) to either archive/back it up or remove it.
In TRS 2006 use CMP to archive/back it up or delete it.
Cheers
VinnyBarb