Prevent bounce message when hourly email limit has been exceeded

I have the hourly email limit set to 3000, which is more than enough for most users. There is, however, one pretty large forum that often goes over the limit. When this happens, a bounce message is generated with an "Unroutable mail domain" error. These messages are causing the mail queue to periodically grow to several thousand messages, and it usually gets to the point where it can take hours for mail to get delivered. The queue returns to a more sane size during the night (when the forum is not as active), and the process repeats the next afternoon.

What I'd like is to discard these messages without generating a bounce message and without them entering the mail queue. Where would I have to modify the configuration to do this?

 

I see, thanks.