We have required to configure the policy to block the yahoo im through DLP?

You can monitor the traffic with DLP:

https://www-secure.symantec.com/connect/forums/im-monitoring-dlp

You can block the application with SEP using application and device control:

https://www-secure.symantec.com/connect/forums/it-possible-block-all-messengers-yahoo-messenger

You can also use the SEP IPS

https://www-secure.symantec.com/connect/forums/how-block-yahoo-messenger-file-transfer

Yes you can monitor traffic through DLP,If you want to block you can using SEP ADC policy.

IM:YAHOO Monitoring on DLP 12.0

http://www.symantec.com/connect/forums/imyahoo-monitoring-dlp-120

Block Applications like Yahoo,Skype,Gtalk etc using Application and Device control policy.

http://www.symantec.com/connect/forums/block-applications-yahooskypegtalk-etc-using-application-and-device-control-policy

https://www-secure.symantec.com/connect/forums/how-block-applications-sep-using-md5