I’ve been making a 1-gallon experimental batch of a lemon/orange melomel with Lalvin 71B, and it has reached the desired sweetness. How do I stop it from fermenting any further? I crushed up 4 campden tablets and added it to the must a couple of days ago, but the airlock is still bubbling away. Any suggestions?
It’s pretty difficult to stop fermentation once it’s going.
Personally, I’d let it finish and completely clear. Then you could add 1 crushed campden tablet per gallon and rack the mead on to potassium sorbate. Wait a few days, then back sweeten to taste with honey. Just make sure it doesn’t start fermenting again after back sweetening it before you bottle it.
+1 to all that.
Try “cold crashing” it in the fridge or snow. If that doesn’t stop it then +2 on adding some potassium sorbate to kill the beasties. 
Just a note… sorbate is useless against an already active ferment.
I can’t count the number of times I’ve finished out, sorbated and then back sweetened.
+1 wait it out, then back sweeten.
True!
My understanding is, the safest way to stop fermentation is to cold crash in a fridge below 40F for a few weeks til the yeast fall out.
Then you can sulfite and sorbate. It’s called “cold crashing” and “stabilizing”.
Or, use an absolute filter of less than 0.5 microns to remove yeast.
You’ll still want to bottle up a few 12 oz test bottles, just to check on them every once in a while.