Need to remove complete ECN task from ECR task change process.

The task is not really on the ECR. It shows there because the ECN is associated to the ECR. If you want not to show it there, you need to terminate the workflow or move workflow to completion. If the ECN is in cancelled state, make sure you not just setting state and moving on. The workflow still remains. If there are end states for your change object, they should only get there by navigating the workflow which should set the state for you. This ensures you follow the process you worked out. 

Hi @AK_10589944,

I wanted to follow up with you on your post to see if your question has been answered. 
If so, please mark the appropriate reply as the Accepted Solution. 
Of course, if you have more to share on your issue, please let the Community know so that we can continue to help you. 

Thanks,
Anurag