That depends on how you became friends in the first place:
1. If you want to remove an in-game friend (one you added in the Find friends tab or by using an invitation code) from your friend list, find that player in the Friends tab and tap X in the upper-left corner of the player's picture.
2. If you want to remove friends from Facebook, first unfriend them in the respective social networks and restart the game. They will be removed from your friend list in the game automatically.

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