This is only a fast how to, so you must know where your squid.conf and other stuff about squid on your machine.
The squid’s acl sintax is
acl [acl name] [type] [type’s option]
Create new plain text file contain the domain to block!
Now add new acl to your squid.conf
acl blockSites dstdomain “path_to_your_file/your_file’s_name”
http_access deny blockites
Ok now restart or reload your squid.