a technique that i picked up here is when stacking the comet-aligned images, make (say) 4 stacks, the first with image 1, 5, 10, the 2nd with 2, 6, 11, etc. the stars should be moved enough between each one that you can reject them more easily in these stacks. then you combine the 4 stacks together and hopefully the stars are gone.
you can experiment with how big the gaps should be to get the best rejection.
rob