If someone writes a check to you and you cash it, can they try to take the money back even though they signed it over to you?

Basically, I sold something on Craigslist and the person paid me in check form. I cashed it and now they want the money back. They were talking about getting cops involved, but I was thinking that it wouldn't work because they signed it to me in my name and everything. Can someone do that?
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