Thanks. I'll give it a go.
As far as can tell the following code should block emails with title of "License Limit Near" ?
[edit] Ah after reading again it seems like it's the template name you have to enter not email title. I don't think it's possible to use the presend as HWMCS isn't using or providing access to the email template in WHMCS.
I HIGHLY suspect they are sending these license emails out via my own server instead of their own specifically so they can circumvent the US, UK and EU regulation around spam emails. They are providing no opt-out or unsunscribe link in these emails and no way to stop the sending of them.
<?php
add_hook('EmailPreSend', 1, function($vars) {
$merge_fields = [];
if ($vars['messagename'] == 'License Limit Near' && $vars['relid'] == 2) {
//Stop the email from sending a specific message and related id.
$merge_fields['abortsend'] = true;
}
return $merge_fields;
});