If they aren't built-in objects, you can just right click the asset in Content and choose Delete.
You don't have to worry about which ones are which, Trainz won't let you delete them.
If you are talking about the menus in Surveyor, the easiest way would probably be to use ratings. In Content you can right click an asset go to Rating and change the rating to something that means something to you.
Then when you are in Surveyor you can filter based on Ratings. For Favorites in Locos/Rolling Stock that really is what Trainz is doing - Favorites all have a rating of 5.
An example is for Tracks, I rated all rail that I'm planning on using as a rating of 1, all tunnels a rating of 2, bridges rating of 3, etc.
As long as I can remember the rating categories, I just filter Track Rating and only those assets will show up.