The subfolders in the
.Trashes-Folder are the Trash-Bins of the users that have logged into the system. Therefore those Trash-Folders belong to the specific users and no one else has a right to access them. So to delete those folders you will have to log in as a System-admin and issue the following command on the Commandline:
sudo rm -rf /Volumes/mydrive/.Trashes/*
That way the rm-Command will be issued as superuser who is alloewd to remove folders belonging to others.
Those folders will be recreated once the user logs in again! So after you log in as system-administrator there will again be a folder
At least that way it works on a default HFS+-Volume. As NTFS is more restrictive with access-rights you might have to use a different approach.
But why do you want to delete it in the first place? As after emptying the TrashBin the folder is empty it is not really wasting space and as it has a dot in the name it shouldn't even pop up in the UI - even on Windows.