Blocking specific sites with pf:
Easiest way to do this is with tables.
# pf.conf
table <blocked-hosts> persist file "/etc/blocked-hosts"
block in quick on $external_interface from { <blocked-hosts> } to any
block out quick on $external_interface from { <blocked-hosts> } to any
# A quicker more efficient rule should just be:
block quick on $external_interface from any to { <blocked-hosts> }
# Now just make a file in /etc named blocked-hosts, and fill it with the addresses you want to block, one address per line.