PF: block out port 80 so that not allow clients in LAN to browse Internet via port 80

[vuthecuong]

Recently I used squid for cache proxy configured  to go internet through 
port 3128.
But internet browser in LAN still connect to Internet through port 80
if in conenction option of Internet browser is chose to connect directly 
to internet,
not through proxy server.
So how can I block out port 80 so that LAN clients must go to internet 
through port
3128 via proxy server?
Tnx in advanced