Subscription spam: challenge question is inefficient

The challenge question is not enforced on expired “link to manage your subscriptions”, neither on the raw “/comment-subscription” slug, without any parameter. So it is completely inefficient in preventing subscription spam.

Subscription spam robots are actively exploiting this vulnerability: I get 4 or 5 subscriptions per hour even with “Challenge question” enabled.

Unfortunately, I had to disable the plugin because of this.

Is there any efficient way to filter out subscription spam?

(Akismet is enabled on the site, but to no avail either)

At least you need to fix the challenge question/answer system: the challenge needs to be present on all forms, and especially when there is no parameter in the “/comment-subscription” URL. Since this is the URL spam robots are using the most.

Thanks