'ello sakuro,
I had a similar issue, and while there isn't an explicit option to disable ammo filling, it can be effectively done by setting the Maximum damage of ammo option in the mod's map settings to 1. This works because the vanilla firearm magazine damage is 5, so should cover everything (unless some mod actually introduces ammo with damage of 1).
Take note that you cannot set it to 0 - since that means any ammo is allowed :)
Best regards,
Branko