I want to delete my all task item, clean all history, but do not need cancel my account, how to do it?
you can delete your completed tasks by clicking on the "Delete completed" link in the bottom-right corner of each project or simply delete all projects.
Unfortunately, it's not possible to clear the "# Completed" history at once, but you can remove each completion event separately by clicking on the "X" icon next to it.
We're sorry for the inconvenience, we will consider adding an option to clear this list in the future. You're welcome to add your vote for this request: http://todoist.com/Vote/showProposal/409/
Also, I will pass this to the developers to see if it's possible to delete the whole completion history on our end, but please confirm if you want the whole completion history (everything that's under the "# Completed" link - all productivity graphs etc.) to be deleted as this action will not be reversible.