Bots of this nature don't typically read the page. They post the known values for the form directly to it.
Which is why they use weird names, so that they know it won't be in use.
Be warned though, that bots aren't the only types of bunk registrations. Surprisingly, there are actually some idiots (must be novice spammers) that will actually sit there and register. Those you just can't do much with, although they are typically a fraction of the overall signups. At that point, it's about "damage control", and having internal practices in place (that you tell no one about) to prevent the "damage" from these types of accounts.
"Damage" can be classified as many things, and isn't the same to everyone or any specific setup. It could be things like showing up in a sitewide feed, sitewide list, or what you feel could be damaging to your site.
Another overlooked option is limiting registration to certain countries. This is provided you have a niche site, where you only desire registrations from a certain country or you could care less about registrations outside of certain countries. This would stop registrations from the typical countries where a lot of the automated requests are sent, unless the site is in one of those countries and isn't blocking it of course.