If you have a need or desire to protect your Joomla web site front-end
and/or back-end by specific IP addresses or ranges (eg; 111.222.0.0/16),
either as a "black list" (disallow) or a "white list" (allow), there is
a small, well written plugin, with no "back doors", that serves this
purpose. It is free and very easy to set up. Don't worry about or bother
with the "table" source unless you really want to build your own SQL
table as a data file to be imported. It is much simpler to just enter,
or mouse copy, addresses into the List Site (front-end) or List
Administrator (back-end) textarea fields.

Ban IP Address/Range for 1.5

http://extensions.joomla.org/extensions/access-a-security/site-access/4201