There are spam filters that you can add. Zenphoto has one included which will do a simple parsing of the message and identify SPAM by its word content. There are also more complex filters--see our web site for third party add-ins. Look for spam filters.
You can also add a Captcha check. This is pretty good at blocking the robot spamers.
Finally, if you have only a limited number of "legitimate" viewers you wish to accept commets from you can set up use accounts and set options so that only registered users may post comments.