some people use different solutions here...
1. Set the "forum email" to "no-reply@yourdomain.tld"; users will usually receive a bounceback (or you can set an auto responder and have email automagically deleted; this would be done through your webhosting provider).
2. Add to the outgoing email with "Please do not respond to this message, it will not be received and will not be responded to; instead please visit our forums".
