Well, he can say it but he has to withdraw it. Here's an odd sentence from the NYT's op-edder today:

Management wants me to make it clear that in my last column I wasn’t endorsing inappropriate threats against Mr. Lieberman.

Here's presumably what he's referring to:

A message to progressives: By all means, hang Senator Joe Lieberman in effigy.

There is no "inappropriate threat" to Lieberman the person here. Just an endorsement, self-consciously hyperbolic, of attacking his public image. Did Lieberman complain?