I'll suggest ,Originally Posted by RobH
(i) Not allowing any new accounts to register for some months ( time-period TBD ) . This coupled with a culling of all inactive ( as well as current spammer ) accounts / should make the site a less attractive target to the spammers .
(ii) Since spammers work on the premise of reaching "reading eyes" / perhaps it's time to close down the site and then reopen it to interested registrants ( under a new banner & URL ). This will shake loose a lot of the detritus that serve only to bloat the numbers "traffic" / helpig to attract the spammers .