I'm sure it will keep fermenting since it has yeast in it. Unless you boil it and kill the yeast.
But if you're going to boil it, why not just boil some water and add a scoop of DME. Then you have fresh wort to add to your slurry.
It seems like a lot of trouble to try to salvage 16oz of already fermenting wort. As Mort noted, you could let that ferment out, decant and add more wort to build another starter.
I just don't see the point in trying to stop it from fermenting.
Took your advice and boiled it. More as an experiment. I froze it and will defrost and attempt to use as a starter for my next ale.