The cold temps in your refrigerator will slow the process down, but it will start to ferment again if you add a source of fermentable sugar, in this case, concentrate.
You cannot really kill the yeast, (without adding so much meta that it would be rendered undrinkable) however, you should always stabilize with K-meta and Sorbate before back sweetening.
Sorbate stops the yeast from multiplying, therefore no new fermentations should occur.
K-meta stuns the yeast.
I hope that this helps.