new user unable to register - says email already registered and it's not

I have set up a membership site using Premise.

One new user is having trouble creating an account and completing the registration process.

we are able to replicate the problem, that wordpress thinks her email is already registered.

Under Users or Members I am not able to find any reference to her email address - and she doesn't appear to have a user account.

I also have a test meal address that previously was an account, been deleted and now I am unable to register with that email....

Is there a way to flush the inactive or deleted emails from Users?

Cheers,

Fred

PS: site is http://www.fmsreliability.com/accendo