MyBB Community Forums

I have a lightly used forum for my motorcycle club.
http://www.highbeemers.org/HBMCforum/
I want to control users, so I have turned on "Administrator Activation".  I want to manually approve all users, which should be club members only.  Total forum users less than 100.
HOWEVER.....  I am getting dozens per week of users trying to join that apparently are created by bots or programs or something automated.
This requires me to search for users awaiting activation, then deleting them.  Takes a bit of time which I would like to avoid.
It has been a while since I set this up, but I am pretty sure I turned on the captcha process to require a human to type in some letters.  But this does not seem to work against these automated users.
I am NOT an expert with all the details of managing a forum.
What other settings can I employ to stop these bogus users?
This is very annoying and takes too much time to manage.
Please be as detailed and specific as possible with any instructions or recommendations.

Thank you so much for your help.

exactly which version of MyBB you are using ?
have you tried Registration Security Question plugin / Signup Questions plugin ..
(I prefer using above 2nd plugin as the first one is very popular and bots might bypass it)

How bout the "Are You Human" challenge plugin?

^ yes, Are you a human captcha plugin can be also used. Javascript Bot Protection plugin might also help.

Thank you both!
I am on MYbb version 1.6.11.
I realize I need to upgrade/update. Please point to a tutorial or step by step instructions on how to do this update. I looked at it once, and it seemed very complicated.

Of the 3 plugins suggested, I prefer the Sign up questions. This would allow me to ask a question that is easily answered by a motorcyclist in our region.
Is the installation really simple? if not, can you provide a link to a tutorial or detailed instructions on installing a plug in?

I am a very competent user of PCs and applications, but administering this forum takes me to a level of detail and complexity that can be overwhelming at times. It feels like you need to be a programmer (which I was a LONG time ago) to take action or to understand all the settings.
Sorry for the rant. I am not angry, just a bit frustrated.

Cheers,

^ as you said, your forum needs to be upgraded to at least MyBB 1.6.16 (version 1.6.17 is releasing soon)
and you might be knowing that MyBB latest available version is 1.8.4 (1.8.5 is coming soon)

installing a plugin and upgrading MyBB - both are not difficult (might be confusing in the beginning)

MyBB upgrading guidance => http://docs.mybb.com/1.8/install/upgrade/
plugin installation => see replies here | see also replies here