I've looked over this request, and I don't believe this can be done for a reason.
The Witch Doctor from Bon Voyage (also where the hammocks came from) uses the hammock as part of the scenario after the Sim greets and accepts his plea for help. If a "no autonomous" hammock mod is made to keep Sims from autonomously using hammocks, this will affect the Witch Doctor. And who knows what will happen when that does happen.
However, if you haven't already, you can look at two mods made by JM Pescado and TJ at their sites. They have different approaches to how the Sim make decisions based on their personalities, motives and logic. Either one will help reduce some if not all of the autonomous behavior with different objects and motives.
JM Pescado's MATYYou can search for the sanity fixes there.
TJ's SimbologyYou can look at either or both the social and play sanity fixes there.
It's a matter of preference what you want from either place.