PHP - How to prevent spam to track all the available email list in the DB?

[lifetube]

I designed a registration form that users can use for registration.When the user registers an account, he/she has to provide an email and then I will use ajax call to a server php script called checkavailableemail.php.If the provided email exists in the DB, then I will transfer back FALSE and the user can see the red warning message and form cannot pass the validation procedure.Here is the question,Is there a way that I can prevent spam from getting all available emails in my DB?For example, the bad guy can design an auto-script that uses some methods to continuously send email validation request. In the end, the guy can get a full list of all my current customer email account.Thank you