The best solution would be to try and direct the wild fermentation into a controlled one. The strange smell ("smells like goat" or garlic or bad eggs or rotten meat, depending on whom you ask) that you have may very well be what's called "Böckser" in German and "goût de bock" in French. I'm not aware of an English translation other than off-flavour. It happens often with wild fermentation.
You can normally get rid of a "Böckser" by oxygenating the young wine and then directing the fermentation into a controlled one.
You might pasteurize the half-fermented sirup, which will prevent further progression of the fermentation, but it will obviously not remove the change in flavor or remove the alcohol.
Better though, store your half-fermented syrup in the refridgerator (or as cold as you are able to, anyway) to slow down/inhibit further fermentation while you're unable to do anything better, and get some selected yeast which you can use to start a controlled fermentation (depending on where you live, you may find it in a gardening or home improvement center, otherwise by mail order).
Now, not everybody is a good neighbour, some will peacefully live side by side, and some won't.
It's the same with yeasts (and bacteria). Killer yeast [example] is sold for exactly this reason (and purpose). You can use any kind of selected yeast, it needs not be "killer". These yeasts usually give a somewhat better taste and they only cost about half as much, too (and are more widely available) but they don't produce yeast-killing toxins.
Preferrably, you will want to use dried yeast since it's both cheaper and higher quality than the liquid stuff. Let a teaspoon of yeast rehydrate in about half a liter of natural cloudy apple juice with a teaspoon of sugar (best and cheapest recipe to make yeast feel comfortable, and virtually doesn't affect taste) and let it in there for 3-4 hours. If you have ammoniumsulfate, you can add a teaspoon of that as well (yeast needs this for reproduction, but if you add none it can usually do with what's in the apple juice, anyway).
Meanwhile, take the cooled syrup out of the refridgerator so it will have a somewhat normal temperature when you put the yeast in (yeast, just like most people as well, doesn't like plunging into the cold).
After those 3-4 hours, you should see a considerable amount of foam on the surface and small bubbles everywhere (otherwise, get refunded, and buy a new package).
Now pour all your syrup into a container with an airlock (glass balloon is ideal, but plastic will do too) and add the yeast. Good luck!