Yes, you definitely can delete old nandroid backups, assuming you know for a fact that you'll never want them back. You can, of course, copy them to your computer, first, just in case. If you're using Clockwork, you can use ROM manager to delete them. If you're using either Clockwork or RA, you can delete them from any file browser (Astro, Root Explorer, etc.). You'll find Clockwork backups in /sdcard/clockworkmod/backup and you'll find RA backups in /sdcard/nandroid/(serialnumber)/. In either of those will be directories for each backup. Delete those whole directories (or copy them to your computer, first) to remove that whole nandroid backup. Be careful not to delete any others or the parent directory.