Well, in theory campden tablets and sorbate together should do the trick. The problem is that neither one of those items actually kill yeast, they inhibit yeast reproduction so I've never actually been able to stop a fermentation with them. The usual way to have a sweet mead is to allow it to ferment out fully, then use campden and sorbate to stop refermentation when the mead/wine is sweetened. This works well.
Of course, if it ferments to dry, the ABV will be increased. But that is the safest way.
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