If the schedule isn't even listed under the "Scheduled Backups" tab anymore you could disable or delete the task directly from within Windows Task Scheduler.
If you haven't already and this solves your problem, I would still contact Macrium as they have directed in case there is an underlying problem they want to address with regard to how this happened in the first place though.
Thanks. I disabled it, it must be a bug. I also sent a ticket to Macrium, but no response yet.