Hello
We cannot require customers to register to the site just to ask questions about the product. But this is only one option to prevent from SPAM, but we loos clients...
I found in this forum way to add captcha in this forum.
Please see it and if it is correct add it in next version with option ON/OFF in VM administration configuration.
http://forum.virtuemart.net/index.php?topic=121238.msg413606#msg413606