What do I use to keep a rusted lawn ornament from rusting any further?

sand off the rust, clean, and use a clear UV sealer

I spray mine with a clear coat of Rustoleum matte acrylic. I let the yard decor rust some and then apply the clear coat annually...3 years and still beautiful without further rust. Hope this helps, and good luck!

Once you're able to clean it up, I suggest bringing it inside for the winter especially in the Northeast.

I agree. Using Rustoleum (you could also spray paint it with color to liven up the color) and bringing the ornament indoors during the winter should help.

Paint with rust oleum paint, I think it’s in a spray can.

Rustoleum paints are great! Here is my project https://www.hometalk.com/diy/outdoor/furniture/bright-cheerful-lawn-furniture-makeover-36592558?r=2

Ahhhh I see :)