You can click on the pack and then the gear. Select Open Folder and it will open file explorer/ my computer and you will be in the mod pack folder itself. Click on the directory before labeled Instance and it will visit here show all the mod packs you have downloaded. Right click the folder and click delete and it will put it in the recycling bin. I tried deleting the pack after and it worked. Maybe that will help.
If you've got a file locked in any program, that generally means the process/file is in use somewhere. Be that a barked Java process or anything else.