The only way to stop a disk zero is to reboot. There is no way to remove disks from an aggregate once they are added and there is no way to shrink an aggregate.
Now given your scenario it might still be possible to reboot and have the aggr add command cancelled out as a result of the reboot stoping the disk zero, but that is something I have NEVER tried and would be VERY cautious about. You could potentially loose data and the existing aggr.
This is a reason why aggr add commands should be very carefully executed.
If you choose to reboot, I would only do so if you have aggr snapshots enabled and present:
snap list -A <aggr name>
This would definately be an unsupport method and if something goes wrong wafliron may be required AND data loss potential is still present.