what contrib folder?
the best and easiest MOD for this is called the RAC MOD.
YOu don't install it using easymod, you just make a couple of edits to three files and you are done.RAC MOD
you choose a special word or phrase for the special code.
you decide where you want to put it so that real people can finde it. ( I put it right on the registration page. ) then you edit the usercp_register.php file with that word ( it shows you where to do it ) you then edit the language/lang_main.php file and put that word or the dircections on where to find the word in the lang file. ( it tells you exactly how to do this )
then you edit the profile_body.tpl file of your style, to put in the new required blank for people to fill in.
that's it, you are done.
no more spam bots.