Elderflower Cordial disaster.

In making a 15 litre batch of Elderflower Cordial I accidentally substituted one of the 100g boxes of citric acid for a 100g box of yeast nutrient. (I was trying an Elderflower champagne recipe at the same time and the boxes looked identical.) Now after bottling, the liquid is going crazy and I'm having to leave the lids on the plastic bottles loose to release the constant build up of pressure. I don't want to waste the batch. Can I rescue it and kill the fermentation process? There's an awful lot of sugar since it's a cordial -- 3 parts water to 3 parts sugar.