A certain player keeps showing up, how can I remove him?

If you feel a player is over-projected, you can edit his projection either in the player profile box or in the table view.  Lowering the player's projection will re-calculate the optimized lineup.  You can also exclude a player from the pool by simply hitting the "x" button on the lineup card or unchecking the player name in the table.  The list of excluded players can be found on the "Excluded Players" tab, in order to add them back into your pool of players, simply re-check the box to the left of their name.