To remove a friend, go to the Friends tab. You can find it in the lobby in the lower-right corner of the screen. After opening that tab, find and tap the Trash Can icon in the lower-left corner. The list of your in-game friends appears. Scroll through it, select a friend you wish to remove, and tap the Delete button.

If the Delete button is grayed out, it means that that player is your Facebook friend and cannot be deleted through the game.