Hi
Most backdoors are progarmmed in PERL so to find backdoors in your server use this command :
locate *.pl | grep /home/ | grep /public_html | more
after, verify all files founded
to block back Connect you must install a Firewall like IPTables
you can find more info of IPtables here :
http://www.netfilter.org/projects/iptables/index.html
and here :
IPTables Firewall Scripts for Linux

wish you benefit