While making lamb and spinach curry, by accident I used sweetened natural yoghurt, instead of unsweetened. How can I disguise the sweet taste? I have added more spices, and also lemon juice.
You cannot disguise or remove the sweetness, once it is in there it isn't going to go away. You can balance it out with acidity, however if it is very sweet then you will need to add a lot of acid, and then the dish will be intensely sweet and sour, which probably is not what you want.
You aren't going to be able to regain the character of the dish, but you might be able to produce something tasty. I would try to add some bitterness, like fenugreek if you have it, and see if that makes it work. There is a point of diminishing returns where you can spend more time and effort adding more and more ingredients to rescue a dish than it's worth, at that point it's better to start over or feed it to people and tell them it's your grandmother's recipe - nobody will ever tell you it's bad then.
Not entirely sure it works for sweetness, it should work for saltiness so it may help here, but add a whole peeled potato and let it cook in the sauce for a while but remove it before it gets so soft it starts to fall apart. It should hopefully absorb some of the sweetness (though you might need to rebalance other flavours as it will absorb saltiness and probably other flavours too).
Alternatively use acid to balance out the sweetness and then some butter to remove the bitterness or sourness (butter certainly works to fix bitter or sour gravy so I don't see why it shouldn't work for this)