Remove some packages from source

Good afternoon,

I have an 12 routers with openwrt, and i created a bash script to automate the build system to compile the firmwares to my routers.
Recently i have a problem with 3 routers because the space on the devices (4MB).
I'm trying to remove kmod-usb-core packages kmod-usb2 -kmod-usb-ledtrig-usbport ip6tables kmod-ip6tables odhcp6c and kmod-ipv6 from config file, to free more space but its fail.
When I use the file generated by diffconfig and run make defconfig, the build system always include this packages.
Is there any way to exclude these packages using diffconfig?