You've been training Aikido for 16 years, and you never learned jab defense? Put up a flat, opposite hand and grip the fist when it touches the palm. Simultaneously grab the wrist with the same-side hand and with uke's fist and elbow in a vertical "tenchi," drop it straight to the ground with your center. Nikkyo and kote gaeshi become readily available, among others.
When people are throwing the jab with bad intentions, I've found this difficult. Any pointers?