The very first step in to be able to delete a folder that is unable to delete is to reboot the server. It should be the most full proof if the folder's permissions are properly set.
You can navigate to the highest point of the file path towards the root, share it out on the network, mount it and attempt to delete the folder in question via a workstation. If the folder is able to delete, you will need to revert the permissions back afterwards. You may reset the permissions in the UI for the Platform Space.
Go within the folder in question and remove files and folders within to see if this narrows down what is causing the folder to be in use and preventing the deletion. Once identified, you would need go into the folder/file's permissions and set it to allow everyone access and attempt the deletion of the items within.
WARNING! The following is purely destructive and there is no restoring the data in the directory you target. Be very cautious when performing this operation.
robocopy "D:\New Folder" "FOLDER TO DELETE" /mir
You need to keep both paths within quotations if they have any spaces throughout the pathname. Be aware this command is destructive and there is no way back from this method of deletion outside of data recovery.