Unfortunately, you cannot delete players from your league.
The reason behind this decision is solely because of the overall stats. If you could delete completely a player from your league, all your past statistics would be biased.
That being said, you can deactivate players from your leagues/seasons.
To do so:
1 - Login to your account.
2 - Click on the "Manage League" tab.
3 - From the left menu, select the league/season you want to deactivate the player(s) from.
4 - Still in the left menu, click on the "Players Management by League & Season" link.
5 - From there, you can uncheck the "Is Active" checkbox of players you want to exclude from your league.
Also note: you can reactivate players who would join back your league. To do so, use the steps 1 through 4. On the "Player Management" page, select the "Inactive Players" tab. From there, you can check the "Is Active" chekbox of players you want back in your league.