I don't know if there is a user interface option to turn off registration email. Maybe you can make changes to the email template instead of write new custom module. You can view/update the DNN built-in email templates by go to menu Admin > Languages, select Language Editor. Look for email templates in GlobalResources file.

If you write create your own rRegistration module, go to Admin > Site Settings then open Advanced Settings section. Update the registration page to point to you new module.

Depending on how you're adding the user, you don't need to send the email. We're just finishing up our own registration module and send our own email (verification step) instead of the core email.

var newUser = new UserInfo
{
FirstName = txtRegisterFirstname.Text,
LastName = txtRegisterLastname.Text,
PortalID = PortalId,
Email = txtRegisterEmail.Text,
Username = txtRegisterUsername.Text,
DisplayName = string.Concat(txtRegisterFirstname.Text, " ", txtRegisterLastname.Text),
IsSuperUser = false,
Membership = {Password = txtRegisterPassword.Text}
};

//By default have their membership denied
newUser.Membership.Approved = false;

//Attempt to create
var createStatus = MembershipProvider.Instance().CreateUser(ref newUser);

//If successful add to the default roles for the portal
if (createStatus == UserCreateStatus.Success)
{

// more code to handle successes or failures below...then we create and send an email asking the user to click a link and verify their account.

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Steven Webster
Manager, Community Platform
F5 Networks, DevCentral