The best way to defeat a Jammer is to understand how they work.
99% of them jam the 2.4 and 5.8Ghz frequencies.
https://www.jammer-store.com/drone-jammers/
So don’t fly using those frequencies would be a good start.
You can build drones to fly using 900Mhz, 433Mhz, 72Mhz and 27Mhz
I have never worried about jamming but I have used all of those frequencies before.
If the jammer is blocking GPS then you have a more complex problem.
You would need an Optical sensor to hold your position.