Probably the only way your going to beat this problem is through approving new members.
In other words when someone signs up to become a new member they have to add a tidbit in a comment section about why they are joining, such as - I am an upholsterer or I want to learn about upholstery. Spam Robots cannot answer questions.

Many sites that do not have a lot of new members signing up everyday have gone to this setup and it has helped tremendously. Only problem is that someone will have to monitor and approve those requests.
I would think that with the small number of new members who are signing up here is small and may benefit from this type of setup. It wouldn't be any more time consuming then searching out and deleting spam.
The only other way around this is by going to a high end forum software package ( which is expensive ). My son is the CEO and principal owner in an International tech company that makes this software. His clients list reads like a who's who. Skype, Intuit, EMI, NBC, BC/BS, NASA, FBI, NFL, NHL, MBA, Yahoo and a slew of celebrities, recording artists and professional sports teams. They also have a data division which provides web hosting, e-commerce and data services to the corporate world.
He has provided all the software and web hosting services for a few of the community forums I have belonged to in the past but he will not " pay the bills " for a forum where the owner also has classifieds and sponsors that provide income for the site. He does a lot of charity work for different organizations through software and web hosting donations and pretty much supports any web site that I ask him to. ( It is part of his Daddy tax package he pays me every year ).
His forum software I know removes 99 % of spam and hacking attempts but it also requires an administrator that knows his way around forum software. Their forum software is amazing and is the Porsche in their industry. Here is their website -
http://www.invisionpower.com/I do not know much about their data centers except that they are located in self contained bunkers in Dallas and Washington DC. The last I knew their reliability rating was 99.8 %.
Unfortunately most free community software programs are targets of spammers. I wished there was a way to rid the internet of spammers. These people do nothing but take up good bandwidth and have no purpose in life but to spam, scam and con people.
Chris
P.S. Richard, they also have an office in London. They have technicians all over the world but their software chief and head designer dude is a bloody Pom.

............lol.....