Make players that have "Big for Club Release Clauses" unavailable
It stinks when larger clubs buy your very good players. To prevent this, select the player you want to make unavailable. See if the "Make Unavailable" icon is available for use. If it is not, simply transfer list the player, then quickly go back into the options. The "Make Unavailable" icon should now be selectable. This will stop big clubs buying your players.