Make an interaction unavalable when tense?

Assuming the file you're editing is of the Interaction class (or a class that inherits from it like SuperInteraction), it should be as simple as adding a mood test to test_globals.

<L n="test_globals">
<V t="mood">
<U n="mood">
<T n="disallow">True</T>
<T n="mood">14645<!--Mood_Stressed--></T>
</U>
</V>
</L>
One other small thing: you may need to specify the target of the test using the "who" tunable. By default it'll check the mood of the Actor (the sim initiating the interaction), but if there are multiple sims involved, you may want to play around with the other possible values for that enum depending on what you're trying to achieve. You can also do a few other things like add a tooltip to explain why the interaction isn't available, etc. How to do all of that's in the TDESC.

Edit: Now that I think about it, there's also a second thing you may want to look into, which is forcing a bail out from the interaction if the sim enters that mood during it. I'd recommend looking at interactions like the do homework one for more info on how that works. For SuperInteractions like that it's really, really easy:

<V n="basic_content" t="flexible_length">
<U n="flexible_length">
<L n="conditional_actions">
<V t="list_reference">
<T n="list_reference">40702<!--ExitConditionList_EmotionStressed--></T>
</V>