but I do think that “punch them in the balls” is obviously impractical enough as not to count as a serious suggestion

Whenever someone tried to do that to me, I took it very seriously indeed, because it wasn't all that impractical. I'd go to great lengths to avoid a punch or kick to the plums.

I'd say a specific and actionable threat should be a prosecutable offence. I suppose it's questionable if Linehan's remark was specific, though it was certainly hate speech. Connolly's tweet advocating burning down a hostel containing asylum-seekers while a hostile crowd was outside, on the other hand, was both specific and actionable. There should be a custodial sentence for that.