I suggest running over to
http://www.nukescripts.net and getting their Your Account module hack. It has just the feature you require. It also has the ability to resend the confimation emails.
OR
You can try opening your
modules/Your_Account/index.php and find this line under the
function finishNewUser part
Code:
mail($user_email, $subject, $message, "From: $from\nX-Mailer: PHP/" . phpversion());
and comment it out like this.
Code:
// mail($user_email, $subject, $message, "From: $from\nX-Mailer: PHP/" . phpversion());
There are more incidents of that line, but only comment out the one under function finishNewUser area.
I think that stops the sending of the Activation email only. I haven't tested it though. Make sure you backup that file first before modifying it.