I think the OP wants to block his users from sending email to domain.tld as opposed to receiving from it.
Unfortunately this is not possible in qmail. However, you can firewall the IP of the MXs for domain.tld and this will/should stop email being sent to domain.tld as your server will not be able to communicate with the mailservers for domain.tld. However, if domain.tld changes their MX IPs, it will stop working.
Technically it is possible to add a domain to an iptables rule, but I don't think a lookup is done on the appropriate IP except for when the firewall is reloaded, so the problem remains.
Oh! Here's an idea! Add the domain to plesk! That way all email sent by your users to that domain will always be delivered locally (to your own server) and since you won't create any email mailboxes, all messages will always bounce. I think this is the simplest solution and should work well.