A lot of the automatic downloads like Pokemon Picross/Pokemon Rumble World and various other junk Nintendo downloads for you aren't fully installed so you can easily "start it up", choose not to download it and then have it remove the icon from your homescreen. If it's something you've installed, you'll have to go into Settings > Data Management and then delete the data. Word of caution it deletes save data so if you spent money or anything, don't expect saves/purchases to be there I think.