What's the fastest way to drop airborne status?

I will assume you are coming back to active duty as a prior service. If so contact your recruiter and work to get those orders amended quick. The fact is that you are not in active duty service until the day you swear your enlistment oath, sign your DD Form 4, and shipped to your duty station or assigned training.

Here's a link to AR 614-200 take a look at paragraph 5-3. It talks about that when you come up on assignment to an airborne unit, you must sign a statement stating that you either accept or decline it. If you decline the assignment, it looks like your airborne status is terminated and the P identifier removed. If you sign that you accept the assignment, you then have to serve 36 months before you can voluntarily terminate your jump status, unless you become physically unable to jump. Otherwise, it looks like you would have to refuse to jump on an actual jump, and then face UCMJ action for that. But it seems from reading the reg that the idea that a soldier can just decide to terminate anytime they want without facing UCMJ action is no longer valid.

https://armypubs.army.mil/epubs/DR_pubs/DR_a/pdf/web/ARN6692_AR614-200_Web_FINAL.pdf

You used to be able to voluntarily terminate. They can’t voluntold you to jump.