To delete the ones you've created, go into your terrains folder and delete the folder with your map name. Also, delete the map file itself from your main terrains folder. So if you were to have a map called "Test" Delete Test.map from FH>media>terrains as well as the folder named after it in the terrains folder. Also, if you have exported it, be sure to go into your exports folder and delete *yourmapname*.fhm