You also need the potassium metabisulfite. Your original question was what to use to prevent further fermentation. Don't use the sorbate without k-meta. It can lead to geranium odors in the wine if you use the sorbate without it.
K-meta is what protects your wine from oxidation. Sorbate prevents re fermentation. Without adding k-meta to your wine your wine could go into malolactic fermentation or worse develop a bacteria that is always present but is in check with the correct level of sulfite.
The kit came with Bentonite, metabisulphite, sorbate, fining agent. At the bottom of the kit directions in step 4- bottling it states that Wineexpert kits contain very low levels of sulphite compared to commerical wine. If you want to age your wine more than 6 months you must add extra metabisulphite to prevent oxidation. So I'm guessing I have to make a trip back to the wine to just buy metabisulphite instead of the Potassium Metabisulfite and Sodium Metabisulfite that I bought...huh? Or would be okay without adding it?
with the kits i have made, using only what they supplied (just like yours did) i have had wines age for several years...no signs of anything going wrong and i never added any extra agents...in all likelihood you have enough now placed in your wine that will protect for the period of time you will drink all your bottles......
some will say add extra agents now but you will affect the taste early on, so you have to be careful......IF you were to keep the wine in a bulk age vessel, then i would feel safer in adding more agents because you could taste test over time to make sure you were not oversulfited affecting taste and could bottle at the right time.
In my view these instructions for extra agents pertains more to bulk aging vessels
potassium metabisulfite is metabisulfite or k-meta. K-meta is shorthand for potassium metabisulfite. K is the elemental sign for potassium and it is quicker to say meta than metabisulfite. Na is sodium so Na-Meta is sodium metabisukfite. Most of us would rather use the k-meta rather than Na-meta for the finished wine addition so that we don't add any more sodium to the wine than necessary.
You have what you need. Please just add 1/4 teaspoon of potassium metabisulfite if you wish to age your wine more than 6 months.