Deny Connection

I have a ASA 5505 config with a DMZ to my small bussiness server. Last nifght my server logged over 7000 failed login attempts to my admin account. The source network, ip and port were all blank. My question is there a way to deny access at the firewall to any connection that does not have an IP? Or is there a better way to block this?

Thnaks

You can allow access from ip-adresses and deny others.
This way Source should be from ip-adress xxx.xxx.xxx.xxx... or else it is denied.