I get
Fatal error: Call to undefined function operate_string() in /home/sitename/public_html/forum/addon_authorize.php on line 48 when i try to reset code on registration.

I get
Fatal error: Call to undefined function operate_string() in /home/sitename/public_html/forum/addon_authorize.php on line 62 when i try to post anonymously.

I get
Fatal error: Call to undefined function operate_string() in /home/sitename/public_html/forum/addon_authorize.php on line 48 when i try to register after filling in the captcha

Please how do i go about this?

You need to upgrade your miniBB to the latest version, i.e. 3.0.1.
The latest Captcha version has been updated for the compatibility with this recent release.

OR, for making things easier, you could upgrade just the recent bb_functions.php, operate_string function is added to there (but I sincerely recommend to install the most recent miniBB).

all done sir, fine and working....... but some bad guys are still posting flood

You can't avoid manual spam, except using other tools like Anti-Guest add-on, or Premoderation add-on.

Captcha is only for protection from the basic automated/roboted spam. When 1000 Chinese come to your website and flood, sometimes only banning the whole IP network may help.

Also, in Captcha, you have options - to enlarge characters amount, change fonts, apply grid, change secret phrase. All of these often help from the automated flood which "broke" the default mode.

Thanks,