This will block blank user-agents and those consisting of a single hyphen, which are "blank user-agent spoofers" -- and even worse than actually-blank ones...
If you don't already have any other working rules, then you'll probably need to precede the above rule with either both of these lines or only the second one. I can't tell you, because it depends on your host's default configuration. You will have to test on your server to find out.
Options +FollowSymLinks -MultiViews RewriteEngine on
7:13 pm on Feb 20, 2011 (gmt 0)
I'm sorry for being a total idiot!
wilderness -- I never even saw the link you posted! Sorry. I thought it was just a sig link. haha. I should read more carefully :)
Jim -- Once again, you are a true life saver! Your posts are always fantastic. Cheers!