Fusion isn't some magic free energy source. Fuel is not the major cost of energy. Infrastructure and maintenance are the main source of energy. Fusion, let alone any free energy source, wouldn't actually lower the cost of energy much. People would still need to have to build desalination plants, power lines, roads, etc. Hydroponics will use just as much if not more chemicals than normal. Vertical gardens and robots are a pipe dream that would cause more enviromental damage in building them than our current growing methods. Energy has nothing to do with people dumping their garbage. We could get the same benefits from nuclear energy if it would actually solve anything.
"Fusion isn't some magic free energy source" - true, but that's like saying that fire isn't some magic heat source... it may as well be for a caveman. Of course fuel is the major cost of energy, see for example coal, gas, nuclear. Renewables are a different case, but the amount of energy is typically orders of magnitude less, so the "fuel" cost (not sure how that would be defined) is negligible compared to the equipment, but there is going to be a maximum amount of energy to be extracted that doesn't scale up - i.e. you can't just throw more coal onto the fire, you're stuck with the amount of sunlight, wind, wave energy, etc. that you have. Vertical gardens would cause more damage than mega-farms? why? Energy has a lot to do with dumping their garbage, although I do think it might have an opposite effect to the GPs suggestion (more energy, more waste produced)