Block Steam's internet access using iptables

block-steam-internet-access.sh

#!/bin/sh
# Block Steam's internet access using iptables
# Script must be run as a user with root (or one that has
# permissions to play with iptables)
#
#
# Why did i need it? Even when running in offline mode
# steam would keep trying to login and then once every
# while it'd end my dota bota match saying "Connection
# Lost".
iptables -A OUTPUT -p udp --dport 27000:27050 -j DROP
iptables -A OUTPUT -p tcp --dport 27000:27050 -j DROP
iptables -A OUTPUT -p tcp --destination 79.140.94.16 -j DROP
iptables -A OUTPUT -p tcp --destination 79.140.94.18 -j DROP
iptables -A OUTPUT -p tcp --destination 162.254.192.18 -j DROP
iptables -A OUTPUT -p tcp --destination 8.254.43.254 -j DROP
iptables -A OUTPUT -p tcp --destination 8.254.130.126 -j DROP
iptables -A OUTPUT -p tcp --destination 162.254.192.16 -j DROP

Tried it on a router, steam login and downloads still work

Given the script was created 5 years ago, I am almost sure that the current IPs that steam client uses have changed in the mean while.
Perhaps try a sniffer to see which IPs steam client connects with and block them?
Or you can run steamclient under a different user and block all network access for processes owned by that user.