Track robotic user

[techiguys]

I have been tracking user with it's IP address but I found that there are number of ip address tracked in some series. I guess it might be because of robotic user. How can I block that user?
For e.g: I am getting the data in this format. Here you can see the series of the data been tracked.
1 United States 74.54.190.74 2009-11-08 02:28:43
2 United States 74.55.248.114 2009-11-08 02:38:44
3 United States 216.166.83.237 2009-11-08 02:48:44
4 United States 74.54.190.74 2009-11-08 02:58:44
5 United States 68.233.164.132 2009-11-08 03:03:45
6 United States 74.55.248.114 2009-11-08 03:08:44
7 United States 208.69.180.109 2009-11-08 03:13:44
8 United States 65.36.241.79 2009-11-08 03:14:12
9 United States 216.166.83.237 2009-11-08 03:18:43
10 United States 208.81.177.178 2009-11-08 03:23:44
11 United States 209.62.120.226 2009-11-08 03:28:43
12 United States 69.12.104.162 2009-11-08 03:33:43
13 United States 74.55.248.114 2009-11-08 03:38:42
14 United States 216.166.83.237 2009-11-08 03:48:43

[crmalibu]

Many robots will obey directives in a robots.txt file. They may also supply a user agent string which identifies them.

[baileylo]

You could also do it via apache
http://httpd.apache.org/docs/1.3/mod/mod_access.html

Or via php, though this isn't the best result, but check the users IP address if it's in the list throw up a 404 error on them.