Well potentially the machines could enter the code, but the codes are random anyway.
Some people use all letters, some use numbers as well and some even use symbols too. Some even use two different words, one of which may just be a collection of random letters, the other an actual word.
The fact is it may stop automated scripts or bots, but it won't protect against human traffic that spams. The only real way I've ever known to deal with some of that, is to block it before it can happen. Source I.P. addresses you know will be likely to used by spam offenders and block them, so they can't be used, pre-empt potantial spam attacks early, when your site is still fresh.
This method is not foolproof however, as you must take care not to block certain networks incase you block legit users, this is dependant of course on which country the network(s) may be and whether it represents a good chunk of your potential user numbers.
India is a pain in the butt, as quite a few indians spam but some don't. Whereas, Korea, Japan and China are almost pure spam users tha\t try to post on sites, and the rest of users from such countries are probably not going to use English, so they're of no relevance to you, in the unlikely event they try to spam, so just blitz the I.P's you find and blitz them good.
It is frustrating when something looks difficult to read, but it isn't that difficult, so if you don't get it once, you'll probably get it at worst second or third time, almost always