As heather says, you must add a moderately large amount of alcohol. (This is called "fortifying" the wine.) This is how Port is made, for example. It is what you need to do if you wish to stop the fermentation early, leaving behind some of the sugar. Generally, people add enough brandy or other spirit to bring the ABV up to about 20%, where no yeasts can really operate. You can use a "Pearson Square" (which can be found by googling) to determine how much it would take, probably something like 1 part brandy to 3 parts wine.
If you just want to have a "normal" sweetened wine, you should let your wine go dry (i.e., completely fermented), then let it clear, then add potassium sorbate and k-meta, then sugar to desired sweetness.
