farmersteve has a good answer, I just want to add some information.
This part of the process is called maceration, it is when you let the skin soak in the wine to retrieve color, tannin and flavour. To prevent fermentation to start (the grape skin contains wild yeast) you may add sulfite and keep the temperature cold.
The "yeast killer" you used, probably contains sulfite and sorbate? Anyhow, the effect will not last, they tend to "evaporate" in time, which explains why your wine is starting to ferment on his own.
Keep it as cold as possible and add your yeast as soon as you get it (and bring the temperature back to normal).