Well.. as far as i know you cannot use squid as offline browsing software , the site is get cached as the site is opened.
Secondly to deny certains sites you have to use the following thing..
Say you want to deny www.yoindia.com and www.yahoo.com and as much you want then do the following
# Defining acl herehere "block" is the ACL name i have chosen, you can use any thing you want.
acl block dstdomain .yoindia.com .yahoo.com .msn.com
http_access block deny