Yes dear cathal
I'm using Google Recaptcha the Strong and free Available recaptcha! BUT It's very easy Traced by spammer registration software.
It mean's this is not strong. Strong means not allow to trace with using some Intelligent questions like: What is 5+21=? or other way for make better captcha an can win in this game.... Google could not stop Spam Registration and We have 10 Register per day in kindcare.ae and I have 100 Website same this and I use Google Recapcha for all of them.
DNN should make different Secure code and don't use CAPTCHA because this kind of secure code is not Secure actually and they will trace easily.
I hop I can Transfer my idea correctly to you. because I'm really confuse also because I don't know why you don't want accept these CAPTCHA is not useful to make secure login or registration and need to creative and new idea to make Special way for DNN Platform.
I love DNN but there is 3 Big Problem for this plat form:
1. It's need to Huge Hardware source like Memory and CPU..
2. It's VERY SLOW in loading regular and busy pages (no different)
3. Easy Traced CAPTCHA and make several Spam Registration even using FREE STRONG RECAPTCHA like kindcare.ae