So how about add real accounts where no one can post without having a validated email address - which can be blacklisted?
And how about add a 'Report' button where it's enough if 2-3 people with 500+ posts press it to auto-hide the post. Or 5-10 people with maybe 25+ posts. And revoked rights to tag threads if abused for other purposes. Maybe even take a chance and let some users alone hide spam threads.
In the end, it's well known that the use of a captcha does not really help - there are too good libraries for auto-processing of them.
Maybe even be mean and requiring that a new account needs to wait for at least 1 hour before being allowed to make any posts to make it less fun for robot software to create an account and then pump in new threads.
This forum can only survive if spam threads are removed fast enough that it isn't meaningful to try to get Google and other search engines to catch and index the spam threads before the treads are dead. Right now, that is a failing battle because too much spam threads survives far too long - making this a top-priority site for adding even more spam.
"also, just making the spam 'read' is not very efficient, that bastards believe that as long as the post is there some idiot will read it"
Not sure what you mean. Of course the intention is to make the posts inaccessible so normal users can't see them - then a Keil admin can quickly browse and verify that the hidden posts should have been hidden and aren't the result of someone abusing their powers by flagging valid posts by someone they do not like.