hi,
you will never have a chance to stop spammers. as they using proxies.. look another softwares.. like forums...
nothing is secure... captcha, reCaptcha.. there is nothing what you can do in my opinion. you banned one ip... they got another one.. you are again banning... new ip... again, again, again...
spammers are everywhere..
only one thing you can do which are you doing already... delete comments... ban user... ban ip (even you are maybe for later banning good users).... as you would maybe on a forum.
that is it.
hard work..