Well, this is a last resort. But you have to remember this every time you upgrade. Reset the option back to "Enabled. Blogs and user accounts can be created." Then in wp-signup.php comment out the Gimme a blog line like so,
<!--<input id="signupblog" type="radio" name="signup_for" value="blog" <?php echo $signup['blog'] ?> />
<label class="checkbox" for="signupblog"><?php _e('Gimme a blog!') ?></label>-->
Then on line 230, change it to
$signup[ 'user' ] = 'checked="checked"';
Now what happens is that when someone visits your signup page, they will only see the "Just a username, please" option which will be already selected because of what was changed above. Once the user activates his or her account, they will be able to create blogs from the buddypress bar because you have the Blogs and User accounts enabled.