ubuntu 18.04 disable ip forwarding permanently

does anyone knows how to disable ip forwarding permanently on Ubuntu 18? I don’t want any device connected via Ubuntu Hotspot to be able to reach any remote subnet, not even the Internet.

I manually edited /etc/sysctl.conf like this: net.ipv4.ip_forward = 0 and reloaded the NT Manager, so I would get

$ sysctl net.ipv4/ip_forward

net.ipv4.ip_forward = 0

After this, I tested the configuration. I created a WiFi Hotspot and I got this:

$ sysctl net.ipv4/ip_forward

net.ipv4.ip_forward = 1

and

$ cat /proc/sys/net/ipv4/ip_forward

1

which is not what I wanted. So, how could I disable ip forwarding permanently?