I've been sitting for the most part for the last 3 or so weeks, and still lost about 10lbs.
I recommend only drinking water for your beverages. I drink about 2 gallons a day (4 * .5 gallon bottle), and I feel it curbs my appetite.
I've also adjusted my diet. Since you're burning nearly no calories while sitting (other than passive calories/calories you're burning so your body functions), you need to adjust for that. The coke is all sugar, which is a lot of calories you aren't burning, so cut it out (cut out all soda). I personally also eat foods that provide what my body needs with not too many calories. I do this with 2% yogurt (***E 2% w/ Strawberry) for snacking, a protein heavy breakfast (eggs/sausage), and plenty of fiber.
I also try to keep a regular sleep schedule, leaving 7-9 hours for sleep a night. Sleep with help with digestion, and generally benefits your metabolism.
Eat small, eat often, and don't waste calories.
This isn't the ideal. Ideally you'd get out and supplement your diet with exercise. But this is a start. Personally, I'm 6ft 210lbs and built like a rugby player. Not everyone can pull off the sedentary life style and still be a healthy weight.