Is it possible to stop more/extra fat from being stored in the body?
Fat storage comes from conversion of glycogen (a glucose polymer) to triglycerides (fats).  (source: https://en.wikipedia.org/wiki/Glycogen).

Intuitively, the simplest way to prevent fat storage is to block conversion of glycogen to fats. Unfortunately, this will cause a glut of glycogen which can cause an entirely different set of problems (see https://www.hopkinsmedicine.org/health/conditions-and-diseases/glycogen-storage-disease ), so you would want to couple this with a mechanism to eject excess glycogen.

Referencing what user1136 posted: uncoupling oxidation from phosphorylation is a really bad idea. That would basically "burn" fuel while blocking the expected ATP outlet. This will cause a dangerous energy buildup in the mitochondria which has to be unloaded somewhere - provable simply by conservation of energy. Energy can be converted to other forms of energy, but it is never destroyed. Physics is the ultimate anti-cheat tool.

Blocking expected energy outlets is almost always a bad idea in practice. Just to name some practical examples:


In archery class, instructors will warn you never to "dry-fire" a bow (draw the string back and release with no arrow). This can compromise the bow's structural integrity, causing it to splinter, snap, or explode later.
Microwave ovens should never be operated empty with no sink for the electromagnetic radiation they produce - this can damage them. See https://physics.stackexchange.com/questions/309154/what-happens-in-an-empty-microwave-oven for more info.
If you jam the pistons in a standard cylinder-based combustion engine (prevent them from moving), you end up with a lot of energy that needs to go somewhere, which will end up as massive vibration and extra heat - either of which could damage the engine as the design never expected this to happen.