You can't do that.
How will you show a reCaptcha in my weechat (or irssi, or mIRC) ? And reCaptcha is for websites. You can forbid access to your applet (if you have one) but not to a channel loaded in it.
You'd better create a script which will show a random string, or ask a question, and kick the user if the user don't answer correctly in a defined delay.
I believe I had such a script ages ago i got from somewhere it worked but haven't used after that as it wont prevent anything abuse or other wise if that's the porpose it's used for