Task coach is great. I needed a cancelled/OBE (overcome by events) state and the solution presented in the comments using the unique category works very well. lb

I think TrungTN got it almost complete (see comment below).
This would be fully functional after this other request is implemented to allow hiding select categories:
http://taskcoach.uservoice.com/forums/26465-desktop-version-windows-linux-mac-of-task-coach/suggestions/563259-show-all-but-selected-categories

Another workaround, similar to other requests about archiving, is to remove the tasks entirely, but you could first archive them by:

Select the tasks and choose File > Save selection…
This will export the tasks to a new .tsk file, and then the tasks could be deleted.
To keep adding to the same collection of archived or cancelled tasks (without having to manually merge every time), there's this other request:
https://taskcoach.uservoice.com/forums/26465-desktop-version-windows-linux-mac-of-task-coach/suggestions/457886-ability-to-move-selected-tasks-to-a-different-tsk

I know this is not for people who want to keep these tasks in their normal list but just marked differently, but some people might appreciate it.

I think the Lukasz comment is reasonable solution.
TaskCoach is flexible tool. You can customize it by yourself.
My suggestion based on Lukasz comment:
1. Create a category "Cancelled".
2. Change the appearance of this category:
- Change the font style. You may check the option "Strikeout"
- Change the icon to "Red cross".
Update your cancelled task(s) category to "Cancelled".
Now your cancelled tasks look great.

Exactly what i am looking for ! - Sometimes a task looked vital at the beginning, but turned out to be obsolete, due to outputs of other tasks. Currently i mark them "complete", just not to loose the details and the findings noted in the task desctiption field - but this is not helpful for future "lessons learned" sessions.

I add canceled tasks to a "Deferred" category and remove start and due dates. I change the color of tasks in this category too to provide a visual difference for these tasks.

Seems very similar to my suggestion of being able to "skip" tasks (i had in mind recurring, but it wouldn't have to be). Should we combine these two feature requests?

http://taskcoach.uservoice.com/forums/26465-desktop-version-windows-linux-mac-of-task-coach/suggestions/289010-add-task-state-skipped-?ref=title

So if we cancel a task, the info is retained, but it no longer shows up as being due or overdue. For recurring tasks, the next ocurrance would still be due at the normal time.

I've been wanting this one for a while. Right now to "cancel" a task I have to choose between deleting it entirely and losing the details or marking it completed and making a note (e.g. in the description) that I "cancelled" the task.