Use iptables to block ssh access to your server

You should read up on iptables first before making any changes to any meaningful servers

http://en.wikipedia.org/wiki/Iptables

To block inbound ssh access to your server do the following

iptables -A INPUT -p tcp -m tcp --dport 22 -j DROP

-A means "Append to INPUT chain"
-p means "protocol" in this case TCP
-m means "load module" i.e. match the protocol TCP
-dport means destination port, in this case 22
-j means what to do if the rule is matched i.e. DROP

Make sure that line is above this line in the configuration as iptables reads from the top to the bottom down:

iptables -A INPUT -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT