I had a Melomel I had to stop fermentation on and stabilize to keep the residual sugars from being consumed. The following method worked well for me so should for you, i'll adjust the measurements for a 15 liter batch.
Once the fermentation has gone as far as you want and you want to keep a particular gravity rack off any lees and throw the Carboy in the fridge as cold as you can get it without freezing. For your size batch it may be good to throw in plastic bottles filled with ice if possible to get that core temp cooled quickly.
Leave in the fridge for 24 - 48 hours.
Add in 4 crushed camden tablets and 4tsp of potassium sorbate. Just make sure it is dissolved well. You may want to draw out a small 1/2cup sample and disolve the stuff in that and then add to the carboy.
I let my brew sit in the fridge another week but I bet 24 hours would be enough. Let it come back to room temp and see if any more fermentation starts up over the following week. If it does you may need to rack, throw it in the fridge again. Allow to clear & finally bottle so you may try bottle pasturizing. There is a great sticky on doing that in the Cider section of the forums here.
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