edit
I’ve managed to implement a solution and it comes in the name of Spam Karma 2. So props to Dr. Dave for the great plugin, all the spam seem to have stopped now and Akismet is picking up even more spam. With Spam Karma working alongside Akismet things are looking good on the spam blocking front. Amen to that.
I find that a range of attacks works best. Bad Behaviour is a good start as it takes down the bots early then a range of filters word blocks and captcha (or not) kill the obviouse spam while blocked IP addresses (or at least flagged) stop the very worst offenders. Follow this up with group blockage (which you have) and an AI like a baysian filter. Anything that gets through can be fed to the filter to make it smarter while the IP becomes a non trusted IP. How much of that you can do due to time or skills is another story although most of that is fairly easy. Try a good quality local “black listing” plug in as a shor fix against IP addresses that are naughty.