Guineas crave greens, it's part of their natural diet. If you can't fence her out of your garden, or protect certain seedlings from her then I'd pen her up and feed her greens/weeds/grass in her pen every day (be sure it's all different greens from what you are growing in your garden tho) and keep her penned until the seedlings are sturdy enough to handle her nibbling. She may also like some fine stem leafy alfalfa hay to peck and scratch thru while she's penned. If she is specifically choosing to eat something in your garden that you fed her as a keet... well, then you kind of sealed your own fate there, because Guineas will often seek out plants they developed a taste for when they were young. She may always go back and devour those plants.
You can also try reflective bird tape, pinwheels that whirl in the wind, pie tins etc to spook her away from the garden area. Motion activated sprinklers may help keep her out as well.