It took some time (almost a month) and really was a battle, but at the moment it looks like ask.libreplan.org can keep spammers at bay. Phew!
No doubt those low life bottom dwellers will find new ways to infest sites, but at the moment the new solution works.
The downside is that I have to sign off on every first post(s) of new users. So, if you are a new user and post your first post(s) it can take some time before they appear. But this seems to be the only solid solution to the spammer problem.