Hacker Scans - Advice requested

[Mike Bruce]

Please can you help me?
 
I am getting increasingly plagued by this message in my security log on
my V4 installations of FreeBSD
 
06:48:53 mail sshd[18617]: Failed password for illegal user admin from
210.3.4.71 port 39741 ssh2 Aug  7
 
Is there any way that this can be prevented without impairing the
services provided by the operating system.
 
Many thanks
 
Mike Bruce