i am 100% positive what ever method we come up with, the spammer will find a way around it. the thinking is to make it more difficult. it is like robbing a bank. i am sure you can think about 1000849380947328798374987329847329874932749327 different ways to rob a bank. should banks stop their process of prevention? no.
You cannot use 'difficulty' to stop an algorithmic bot, and any attempt to do so would have zero lasting effect on the total amount of spam. A bot does not get tired out, it has no frustration, it has no impatience, it exists solely to make posts.
Everything is about risk vs reward, and at the moment these spammers do not post an existential threat to the forum. When the spammers come, the mods quickly clear them out, and everything moves on as normal.
Adding arbitrary restrictions poses no 'reward' whatsoever, because the bots can easily bypass all the restrictions with no effort. But the 'risk' side of the equation increases exponentially due to new members getting wrongly flagged and frozen.
At the moment, the spammers are only inconveniencing the mods a couple times a day. Punishing all the new users isn't going to change that at all, it's just going to leave the mods to deal with the exact same amount of spam as before, AND a lot of people complaining that they cannot post anymore.
Or to reuse your bank analogy, if the robbers are coming in through the roof, placing landmines in the lobby isn't going to help anyone.