I wrote in Battletoad, but I was also quite relieved to see that Serenity was winning.
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Just tell them you'll be having friends call periodically posing as potential employers. Let them know that if they say anything untoward about you while providing a reference, you'll see them in court.
For bonus points, mention that suing their asses for something as petty as that will likely be more profitable and less work than having whatever job they're trying to torpedo out from under you.