I had a peach wine that had too much sulphur in it when fermentation was complete. This is because I had preserved the peach juice with metabisulpite. The result was a rotten eggs smell.
I tackled the problem by adding MORE metabisulphite. This precipitated out the sulpur that was causing the bad smell. Still some smell so I added some copper to the wine - short lengths of copper pipe - and within a few hours the smell had gone completely.
Not quite the same problem as yours but the copper might work. I wouldn't add more metabisulphite because it's a different problem. When I have had a similar fermentation problem due to too many preservatives, usually potassium sorbate, I have started the fermentaion again with just the sugar element and once fermenting vigorously I added the rest of it a litre at a time. That way the yeast was able to overcome the preservatives.