Somebody's IP is blocked... How do I unblock the ip address they have?

Well that would depend on how they are blocked!

There are many ways to block an IP address:

.htaccess control restrictions

blocking in the httpd.conf

hardware firewall

software firewall

mod_security

hosts.deny

proxy or network restrictions on the visitor
on the visitors own computer or ISP.

Bad firewall settings on the visitor's
own computer at home.

You need to know what the blocked visitor is actually seeing
when they try to connect and you need to review your log files

 

The most likely way they were banned would be through one of the common firewalls - either CSF or APF. With APF, you edit /etc/apf/apf.deny (I think) and restart; with CSF you can either edit /etc/csf/csf.deny and restart or do it through the WHM gui.

Both of these firewalls ban people who retry passwords multiple times, so that's the most likely cause of the bans.