It's almost impossible to stop a fermentation once it's going. It's like trying to stop a freight train.
What most mead and wine makers do is to let it finish. When it's completely done, completely clear, and ready to bottle, then a campden tablet (one per gallon and crushed) and potassium sorbate is added by racking the mead into it. Wait a few days, then sweeten to taste with sugar, honey, brown sugar, etc. It's easiest to do it if you pull out a sample with a wine thief or turkey baster, and then dissolve the sweetener into that. Add it to the carboy, wait a few days to ensure that fermentation will not restart, and then bottle.
Some have stabilized (added sorbate and sulfite) before allowing it to finish, with mixed results. It's always best to let it finish, so you know that you won't have it restart in the bottle, and then sweeten it.