I suppose, by it's very definition, anything that makes you lose weight quickly is going to be a temporary thing, basically a crash diet. It will leave all your bad habits, which made you overweight in the first place, unchanged. So when the crash diet is over, it all comes back again.

Having said that, there is one way to lose 30lbs in a week that is absolutely guaranteed, and you'll never put it back on – amputate your legs.

Do exercise daily
Consume Fruits and Vegetables
Stay Hydrated
Intermitted Fasting
Low Carb Diet
Keto Diet