Install macOS onto a flash drive, boot from there by holding Option at boot up. Browse to the user on the computer, hit Cmd-Shift-. to reveal the hidden Trash folder. It will be restricted. Right click, Get Info, unlock it with the credentials of the boot flash drive. Click on the gear to add a user to access it. Choose the flash drive’s user. Set the permission to Read & Write. Now you can access that Trash folder and empty it. Then restart the computer back into the default user. Trash icon will still looks full but just empty it again.