Sometimes you will want to remove unwanted players from your table during a game with other people.
If you are still seeing the available tables window, then you can remove the player in that window.
If the game has just started, you can still remove players that came to the table after you. Before you have ended your first turn, you see a deletion button (depending on the theme, but usually an 'X') next to the player's name. Click the button to remove that player from the table.
During a game, you can only remove a player when that player hasn't played for two minutes while it is his turn. In that case, the removal button appears again next to the player's name, and you can remove the player.