1. If you installed via steam, just unsubscribe fromt them and they will be removed from mod list.
2. If installed manually they are located in C:\Users\*Username*\Documents\Endless Space 2\Community\
3. Worth mentioning that mods are not automatically installed and present in the game after subscribing and/or installing localy. You need to activate them in game via mod menu and only after they will take effect in the game, so if you played a game and wish to turn of mod, it can be done same way as activated via mod menu in main menu screen.

Tank you. I unsubscribe and the mod is indeed uninstalled. And thanks for point 3: well taken.
Thanks