Are there any plugins that prevents bots from entering my forum?
Well, do they have something more secured?
Then develop yourself, because no one had ever tried for such thingy for free.
I recommend installing Tim's StopForumSpam mod.
the questions things never worked for my sites very well, tried it on my vbulletin forums and mybb forums., it slows them down but doesn't stop them. i use to use stopforumspam, but the issues i was having with it was, it was blocking everyone...(was funny I couldn't even register to my own site), too many legit people which i couldn't deal with that..seems to be a big complaint if you seach google... i've been using fassim plugin and it doesn't get everyone but works a lot better for me. i think a combination of plugins is the way to go.
Well, using Captcha is the best solution to stop spam registrations. If you mean to stop "spa posts" then use Akismet.