I keep getting spambots joining the website and advertising a bunch of garbage, How do I add a captcha to my websites registration? I added security questions, but they don't seem to be helping.
...I'd suggest that you are prompting them do that by allowing their foolish/childish nonsense to appear on your system. For each one you display you are probably attracting many more of them, as they can see you do not have any systems in place to prevent people joining and posting SPAM/malicious links.
Having the messages of new members moderated is one simple step that would mean another such message could never appear live on your site again.