Jelle-B idd, there are so many solutions. but they do the easy way again, and that means shaving a whole bank over the comb. idd a maximum of eg 3 free per address is the best solution, plus just use pc + house number db that filters out strange characters or additions to a zip code and this will result in an error on the user's side during registration, so that also that loophole is closed.
With regard to Germany, idd, there is apparently at present a form of IBAN discrimination at OTA, because I also hear through the various German ( bunq ) social media groups that it is difficult or impossible to go to the German site with German address in combination with bunq , that does not belong and they have to solve it as soon as I am concerned.
And as far as possible German fraud is concerned, there too can be a maximum per address plus check on house number and PLZ.