I don't know, most of these applications now come with a list of client types that you can choose from right in the most basic config. Some are even sophisticated enough to follow javascript links and submit form data. Trying to block these is only going to stop stupidest users.
A better (but much more complicated) solution is to check the rate of request from every user and when they exceed your limit, redirect 'em, or even suspend the account. That should shut down everyone except a real surfer.
Good example is GFY search function, they have it limited so you can only do one search every 20 seconds, otherwise it makes you wait.
