According to the manpage of "mount" and "fstab", "defaults" includes "nouser", so "nouser" is superfluous. However, "defaults" also includes "auto", which will mount the partition at boot time. Guanx solution prevents this behaviour. You might also want this partition to be mounted read-only via
/dev/partition /mountpoint vfat defaults,noauto,ro 1 2
so that mounting the partition in rw-mode requires explicit specification. This will force the superuser (i.e. you) to think twice before doing any modifications to this partition.
For more information, see
$ man fstab
$ man mount