Yes, sorbate (potassium or sorbic acid) will prevent yeast from reproducing. The only way to ferment this batch will be to inundate it with massive numbers of live yeast cells. How big was the batch, and do you have any more grapes?
If you have more grapes, start a batch with those grapes at least half as big as the sorbated batch. When it has really taken off, add the sorbated must to it. Be sure to get ALL the lees, as these are your active yeast cells. The combination will ferment slower than normal, but it should complete.
In the meantime, tightly seal and refrigerate the sorbated must to prevent non-yeast spoilage organisms from gaining a foothold. Return this must to room temperature before adding it to the fermenting must.
If you don't have any more grapes then your first batch will have to be a blend with a similar kit. The same procedure applies - start the kit and when it is really going add the sorbated must.