by sigmazero13
To be precise, the rule for this is that when you RECRUIT (using a Recruitment order or cards that say to recruit units), you can remove units from the board if you've reached your limit.Other abilities (like the Necromancer one, stated above) are not "recruiting" and thus do not allow you to do this.
Note, however, that you cannot just scrap a unit anytime you want. You can only do it if you run into your plastic limit and want to recruit a unit.