Bananas are wonderful. They are not a bad food.
I've heard this a lot, bananas stop me losing, grapes stop me losing, muller lights stop me losing..... too much food when you are not hungry stops you losing.
I eat one a day, usually with breakfast. They are good for you. Like with all foods, don't overdo it.
If you have a fab loss one week by cutting them out, I doubt it's bananas alone - let's face it, even at 3 a day (300 calories) x 7 days, that is 2100 calories - hardly a massive weight loss.