I have the same issue as you, but I haven't stopped them nursing.
My cat has lost so much weight due to the feeding and I'm feeding her almost 6-8 times a day. Her kittens eat too, but it's breaking my heart to see mom so hungry all the time. I over feedoverfeed because she is looking thin.
We went to vet today and she too is concerned and said to shoo the kittens away if I see her nursing. So, so here it goes.
So no I don't think it's cruel or otherwise to stop kittens nursing especially if they are feeding normally too. Take
Please take care of the cat mommy, too :)).