Except for some failed attempts to get it work on my more modern PC, it's been years since I used Radiant, so I can't guarantee a 100% perfect answer. So, if you want to remove vehicles, you have to remove the blocks that represent the vehicles (the blocks are like visual stand-ins for the models you see in the game; they are usually red and for both NPCs and vehicles). Try to locate the vehicle, select it and remove it. I think you have to use either STRG + Left Mouse Button or Toggle + Left Mouse Button (or was it the right button?) to select something. Once you've selected the vehicle you want to remove, press backspace.