OK, pick a large file you want delete and run:
"echo > file_name" (for example: "echo > /mnt/Backups/Windows/SERVIDORES/10.1.0.1/Documentos/Hacienda/Documentos_2013-09-18.RAR")
This will truncate the file to zero bytes, freeing some space, which should allow you to delete the rest of the files.