And to head off the "any noun can be verbed!" and "language evolves, get over it!" guys
If you're comfortable with "crackdowned" and "crackdowning" then by all means, feel free to use "crackdown" as a verb. But if you find yourself saying "cracked down" or "cracking down" then you must admit that the verb is two separate words: "crack down."
See also: shutdown, backup, standby, etc.