We've seen a dramatic increase of the spam that we get on the forum lately, and I am wondering if it isn't the right moment for increasing the complexity of accounts creation. Would it only be with two captchas, that could pretty easily be implemented : one graphical and why not, as we are on Mathworks, one mathematical (e.g. basic arithmetic with a few levels of parentheses, enough for screwing up most regexp-based parsers)?
EDIT - some arguments from my comments below
I don't really care about clicking on multiple threads to see whether they are spam or not. What I do really not like though, is to have to do it multiple times for the same user. I think that any solution which allows a same user account to send multiple posts after one has been closed or flagged as spam by a high enough rep. member, is not adequate.
If I look at the list of members with a rep above even 1000, I don't see anybody who should legitimately not be trusted about using adequately a mechanism which allows to block an account until a high rep member (2k+, 3k+,editor,admin?) decides to unblock or to cancel/delete it. This mechanism could even be limited to accounts with a rep below 25 or 50, because no spammer will ever pass this threshold (see Jon's comment about this point).
EDIT - There could/should also be a list of forbidden domain names for creating TMW accounts, typically those which allow the creation of anonymous/disposable accounts automatically. The rational is that no professional/student should have any valid reason for using such email accounts for creating a TMW account.