Your options to backsweeten are as follows;
- Increase the alcohol content past the tolerance of the yeast. This will result in a beverage of 12% or higher ABV.
You can do this by starting with a higher initial gravity (this is my approach for sweet meads), or adding sugar until the fermentation stops restarting, or adding neutral spirits to bring the abv up, then backsweeten.
- Ferment to dry, then add sulfites to prevent the yeast from reactivating, then backsweeten as desired.
Warning! This doesn't always work, and it's hard to predict how much sulfite is necessary, if you add too much it can negatively impact the flavor.
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Ferment to dry, then use filters down to 1 micron to filter out the yeast, then backsweeten as desired.
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Ferment to dry, then backsweeten just before consumption.
Most commercial producers filter and add sulfites.