How do you did it? I tried it several times, but haven't got how too.
You have to edit the free play parameters. You can find them at your Specs folder (inside your modfolder, if you want to mess a mod's parameters) they are in a xml file called fp_params_endless.xml. Open this in notepad. Here you find the fire events in endless free play.
Find some fire event and if you want to disable it completely (like i have done with earthquake on some mods) change the <Enabled value = "1" /> to <Enabled value = "0" /> and save!
I would not recommend you to disable the events completely, just edit the <AverageFrequency value = "0.4" /> to a smaller value and this specific event will occur more seldom.
REMEMBER TO BACKUP BEFORE YOU EDIT THESE FILES!!!
Here is a link where you can read more about this. How to edit freeplay files by MikesPhotos.