Go to your Documents\My Games\GalCiv3\Mods folder. Find and delete the subfolders for each mod you installed. The folder ExampleMod comes with the game, so you don't need to delete that, but all other folders are the mods you installed.
If you have the Crusade expansion, also check the Documents\My Games\GalCiv3\EXP2_Crusade\Mods folder. If you installed any mods there you should delete them also. Again, there may be an ExampleMod folder there, leave it alone as it was shipped with the game.
Note that at game startup the XML files of the mods are compiled and bundled with the saved games, so to see the effect of deleting your mods you'll have to start a new game.