De-activating and re-activating a user

De-activating and re-activating a user

I de-activated a user of Zoho CRM and now need to re-activate that same person. When I add him as a new user I get a message saying that the email is already, specifically: " This Email address is already taken. Specify a different Email address for the user". 
How do I completely erase a user from Zoho so that I can re-activate the same email account?