You've got it right. Derek Sivers recommends doing this for any serious rentacoder/vworker/getacoder projects you post:
"""
At the end of your post, write something like, “VERY IMPORTANT: To separate you from the spammers, please write I AM REAL as the first line of your bid. We will delete all bids that do not start with this phrase, since most bidders never read the requirements. Thank you for being one who does.”
"""
Of course it's not a sufficient criterion by itself; Sivers' point was that a lot of them won't even do this, so you can at least save yourself the trouble of evaluating them.
""" At the end of your post, write something like, “VERY IMPORTANT: To separate you from the spammers, please write I AM REAL as the first line of your bid. We will delete all bids that do not start with this phrase, since most bidders never read the requirements. Thank you for being one who does.” """