You can use iptables to block connections, by example, using ufw the uncomplicated firewall:
# apt install ufw bind9-dnsutils # Install ufw and nslookup
# uIPs=$((nslookup reddit.com && \
nslookup www.reddit.com && \
nslookup redd.it && \
nslookup redditstatic.com && \
nslookup www.redditstatic.com) \
| awk '/^Address: / { print $2 }' | sort -u); \
for uip in ${uIPs}; do \
echo -n "${uip} " && \
ufw deny from "${uip}" comment 'deny reddit.com'; \
ufw deny out to "${uip}" from any comment 'deny reddit.com'; \
done;
# ufw enable
# ufw status numbered









Also, to block them in the firefox browser, you can also use the
FoxyProxyextension, and use the proxy by patterns options to use a non-existant proxy for the domains that you want to block.Edit to add config example: