Specify an empty string for the redirectMessage. You can play with this by editing:
XenForo_ControllerPublic_Post::actionReport
If you replace this:
Code:
$controllerResponse->redirectMessage = new XenForo_Phrase('thank_you_for_reporting_this_message');
...with this:
Code:
$controllerResponse->redirectMessage = '';
...then reporting a post will be instant with no redirect message.
In my testing I found that you must explicitly set an empty message. It is not enough to simply not specify a message because the renderer will use a default redirect message.