I am always hesitating between erasing a canceled task or mark it as complete (adding a note that it is not) for it to stay in the exchange db for historical purpose

If i do not mark it complete, the other choices are

• deferred
• waiting on others
• in progress
• not started

the problem is that if i choose deferred or not started, it stays in the active tasks pool.

So my proposition :

Add a cancelled task status

Any toughts, ideas or explanation ?

If you need to keep it, then mark it complete, otherwise delete it. 

As for adding another task status, if they get enough requests they might add it in a future version. Create a request at http://office365.uservoice.com/forums/273494-outlook-and-calendar so others can vote on it.