You can block websites from the proxy users, just create a separate file that will be the list of domains you want to block and point that file from the squid configuration,
sudo touch /etc/squid/blocked
Then type all websites you want to block one per line in that file and save it.
Change the squid configuration to block those websites under acl list and http_access list.
acl blocked_sites dstdomain "/etc/squid/blocked"
http_access deny blocked_sites
sudo systemctl restart squid
There are a lot of ready to use lists on the web and they are categorized, you can use them in squid, like MESD blacklists, Shalla’s Blacklists.