Answer:
Tell Jason he is not interested in joining.
Step-by-step explanation:
The best answer to Jason is to tell him that Emmanuel is not interested in joining, this way Jason will just back off. Maybe he'll push Emmanuel, but if he talks to Jason with conviction and decisive tone, Jason won't try again.
On the other hand, other options are not precise or right, because they can triggered Jason with an aggressive stand, or just make Emmanuel be seen as undecided, from which Jason will take advantage.