Is there a way to disable the trunk button on the key fab?

Unfortunately there is no way to disable just one button (unless you physically pry it out of the fob). All you can really do is try to be more careful when pushing the buttons.

I had the same problem with my panic button. I simply cracked the key fob open and sanded down the red nub for that special button. Now you have to push it REALLY hard to get it to go off. The other three buttons, including the trunk button are different, but you could still sand down the little black tactile button nub a millimeter or so to make it require more pressure to activate. Take a little at a time and test it until you have it right. IMO the buttons on these Nissan keyfobs are way too easy to actuate, but they sure look sharp huh? Form should follow function, but by the time you find out you've already bought the car amirite?