Disable duplicate shopper login

[Maggy29]

Joomla 2.5.4.
VM 2.0.6.

While testing the shop, I found that when registering on check-out with the same username as an already existing one, I only get an error message, saying "User name in use", but the user in created anyhow.
The same happens when an invalid e-mail address is entered: an error message is displayed, but the user is creating anyway.

Is there a solution for this problem: I think the user shouldn't be created when the entered data is not correct.

[Maggy29]

After some reading I understand that VM registration and Joomla registration are being used together and it looks like this problem is within the Joomla registration functionality. As far as I can see the VM registration only checks for required fields and leaves the checking of the content to the Joomla registration module.
Normally I would expect that a user cannot submit his registration form, before all the entered data is correct.

Is there a way to solve this problem?