If a quarantined message is not required, can it be flagged as such and deleted, and if so, when deleted is there some form of blocklisting done on the senders credentials?

Apart from offering quarantine as an abuse handling action, MailChannels supports rejection and flagging.

When a message is rejected, the originating MTA will receive a negative SMTP response which in turn should cause it to notify the sender via NDR message. This should all happen without intervention by the receiver; however, the message will show up in the receivers logs, where the sender can further be blocklisted.

When a message is flagged, MailChannels adds a customizable header to the message which the receiver’s mailbox server can use to file the message in the user’s Spam folder. Again, this message will appear in the receiver’s logs, where the sender can further be blocklisted.