She thinks you are playing. I am guessing you want her to drop what she has and then 'leave it' alone. These are two separate skills. I would teach 'drop it' or 'give' first. Dont try to take ('steal' - in her eyes) stuff from her, you want to teach her that it is safe to give up things she has. If she has a toy in her mouth, stick a treat in front of her nose, say'drop' or 'give', when she does, say 'yes' and immediately feed her the treat and give her the toy back. Practice lots, this will teach her to 'trade', that giving up something she has means good things happen, and she will (more often than not) get it back . This will come in very handy as she gets older and is trotting around your home with your favorite sneaker in her mouth.
'Leave it' or 'Off' - means 'don't touch unless or until I say you can'. To teach 'Off' (leave it). First teach 'take it' Prepare yourself with a handful of tiny treats. Offer her 3 or 4 treats one at a time, saying 'take it' as she does. Then,with a treat in your closed hand, hold it out to her, and say 'off', hold your hand still,as soon as she stops trying to get the treat say 'yes', open your hand and say 'take it'. She 'understands' when you hold out your treat hand, say 'off' and she doesn't try for the treat. Remember to say 'yes', when she stops trying to get the treat, then 'take it' and give her the treat. Once she is good with the closed hand, practice holding the treat out in your open palm, quickly close your hand if she tries to get it, but don't pull your hand away. Gradually increase the time you wait before you say 'take it'. Practice in short sessions, start each seassion with 3 or 4 'take it's and vary the time you ask her to wait before saying 'take it', during each session. In the beginning, wait just a couple of seconds before saying 'take it' and slowly increase the time you want her to wait, but don't always make it harder. This teaches self control, with practice and over time she will understand - when you say 'off' (leave it) she is not to touch, until/unless she has permission. Practice, patience, and more practice.
Rescuing one dog, may not change the world, but it will surely change the world, for that one dog.