You can use the Q&A CAPTCHA as a registration code.
Q: Type in the Registration code.
A: the registration code you have provided
Then, email the registration code (any string of numbers and/or letters) to the prospective registrants.
How to configure Q&A
- Administration Control Panel (ACP) > Spambot countermeasures> Installed plugins > click the dropdown box and select Q&A (it will be greyed out)
- Click the configure button then click add
- Add your question and answer (you need a Q&A for each language you have installed) click submit
- Click "back to previous page" then click "back"
- Select Q&A from the dropdown box (again)
- Click submit at the bottom of the page.