When you remove a folder or file, it is actually moved to the trash. As long as it is in the trash, it will still be included when calculating the disk usage of your Lab.
Only administrators have access to the trash, which can be found under Settings > Trash, and have the ability to permanently delete files or restore them. If you do not have administrative rights, please contact your administrator to remove the files from the trash for you.
After emptying the trash, it may take a few minutes before the freed disk space is available.
Automatic clean-up can be enabled in the general settings to ensure files are removed from the trash periodically.