When the ball is on the ground a player is allowed to block/obstruct an opposing player from getting to the ball with the body, it's called boxing out and completely legal, the ref was wrong and a bad call.
Here's the rule, in its entirety:
q. Detaining: Detaining an opponent at any time by holding or pushing against her body, clothing or crosse with an arm, leg, body or crosse. A player may not hold her crosse in such a manner as to restrain or hold back a player.
You have the wrong sport for "boxing out". That's basketball.
