Hi, I have four exchange servers, 2 mailbox servers in a dag and 2 CAS servers.

users are receiving multiple advertisement and junk emails.

I block one by one using set-senderfilteringconfig

how to eliminate unneeded emails from Exchange before it reach user mail

Hi 

Better enable all the anti-spam agents in exchange 2013

You can follow - 

http://www.msexchange.org/articles-tutorials/exchange-server-2013/security-message-hygiene/anti-spam-and-anti-malware-protection-exchange-2013-part1.html

In addition to this i would recommend you to bring up any antispam filtering solution on your on premise or any cloud filtering antispam solutions for best industry practice recommendations

Hi,

all anti spam agents are enabled in Exchange 2013 organization.

Hi 

If thats the case then you need to check your email routing part thoroughly

Make sure that your domain is not in a open relay.

Check your receive connectors configuration once again and make sure they are not in a open relay

Configure reverse DNS for your SMTP connections

Configure SPF record which will avoid spoofing your domain

Also if you are using only Exchange 2013 antispam then i would suggest you to follow Best practices from Microsoft

https://technet.microsoft.com/en-us/library/jj218660%28v=exchg.150%29.aspx?f=255&MSPPError=-2147217396

Good Luck