In order to adjust your exposure to certain players while using the Lineup Optimizer, you can set minimum and maximum exposure percentages right from the player pool screen.
When creating multiple lineups at once, you can adjust the maximum exposure your lineups have to a single player by moving the slider to the desired percentage as seen below.
Lastly, if you'd like to limit your exposure to certain positions while using the Multi-Lineups Optimizer, you can do so by clicking on the Set Position Maximums link as seen in the screenshot above.
This will display percentage boxes for P, C/1B, 2B, 3B, SS, and OF that you can manually adjust as needed.
Tip: If you limit an individual player's exposure from the Player Pool page, this will override the position percentage limit and the global percentage limit.