I'm tempted to point out that this mainly affects new editors who cite their edits, other new editors will get bitten in other ways. But the internet is not the best venue for irony.
More practically, if you have a tame admin on tap then you can reduce this and other problems at editathons by setting those new accounts as "confirmed". And yes I know we also have a shortage of admins, and also that it is likely that only a tiny proportion of the editors we lose through this are at editathons.
Earlier this year as a result of the glam organisers event in Paris I made a proposal at bugzilla for an event organisers useright. This would have allowed us to circumvent this problem at those editathons that are targeted at newbies, and it got widely endorsed by GLAM editors from several languages. Sadly it got marked as resolved because there was something that looked similar to developers, though not of course to potential users. If anyone here knows how to bypass phabricator or how to mark a phabricator request as unresolved and still much wanted, then the link is https://phabricator.wikimedia.org/T91928 alternatively perhaps we could persuade the education community to endorse it, it should be just as useful to them and they seem to have more clout with the WMF than the GLAM community.
As for whether the capcha is useful in keeping out spammers, remember there are two capcha steps, one when you open a new account and the other when you use that to add links. Presumably any spam program that can pass the first hurdle can pass the second. But for new good faith human editors each capcha is a possible lost edit/editor. It would be good to test dropping the capcha requirement for adding new links, alternatively perhaps we could whitelist certain domains as likely to be reliable sources and unlikely to be spam.
Regards
Jonathan