When you no longer need a resource, you can delete it.
To delete a resource:
From the project navigation in the Dashboard, select the project which contains the resource you want to delete.
Click on Resources.
Click on the resource you'd like to delete.
On the right side in the header, click Settings.
In the popup window, click Delete resource.
After deleting a resource, any existing translations will still be available as Translation Memory suggestions as long as the project exists.