Thanks for replying.
As Marcus suggested, I added the following lines to squid.conf:
acl blockanalysis01 dstdomain .scorecardresearch.com .google-analytics.com
acl blockads01 dstdomain .rad.msn.com ads1.msn.com ads2.msn.com
ads3.msn.com ads4.msn.com
acl blockads02 dstdomain .adserver.yahoo.com
pagead2.googlesyndication.com
http_access deny blockanalysis01
http_access deny blockads01
http_access deny blockads02
But, tried testing blocking other sites to see how squid handled it:
acl blockads03 dstdomain .msn.com
acl blockads04 dstdomain .testsite.com
and I'm able to access these sites. Shouldn't these be blocked?