How about some data?
That's kind of a joke. In my half-assed attempt, I wasn't able to find much data. I believe the effect was minimal. I don't buy that these hypothetical people exist - the ones who originally thought, "Cleveland sounds like a nice place to go," then, upon hearing that LeBron James wanted to work and live elsewhere, thought, "Wow, that Cleveland place sounds awful. I'm not going to go there."
Also, the economy was already in the shitter at the time, and didn't get worse as far as I can tell.