Gary --
Two ways that I can think of --
The easiest is to use TrainzObjectz.
The second is to find the folder containing the scenario in your Dispatcher folder, and just delete that. Search in Programs Files/TRS2004/World/Dispatcher for the folder that corresponds to the kuid you downloaded.
Remember to also delete the file world_cache_Dispatcher.chump in the Cache folder for the changes to take effect.
Phil