If you want to keep the flavor mostly intact without it getting muddled or washed/fade out, I would suggest 1 or 2 drops of white vinegar (for bakeries/creams), apple cider vinegar (for fruits and lighter creams), peach/pear/guava/citrus in general, any sour/tart/bitter flavoring since it will cut through that sweetness or saline. I wouldn't suggest mixing it in the original bottle, but a 10-30 ml bottle you might have flying around will do. Just try a small batch and see which one does the trick for you.
All of that will reduce the sweetness on your taste buds but not the actual "sweetener" content that's in the juice. Diluting it will not necessarily make it less sweet, it will just lower the flavor overall, sure it could then taste less sweet but you might also get no satisfaction from it, especially if it contains nicotine which would be lowered as well. Adding even more sweetener/creams/marshmallow and what not to it, is a great suggestion but since its store bought I'll assume its at the maximum flavor strength, and would only result in muting it as well as decreasing the shelf life.
In general its extremely difficult to remove additives from juices, especially commercial, and as always there's many roads that lead to Rome. All these suggestions might not do anything depending how sensitive to sweetness you're. You'll never be able to completely eliminate the added sweetener.