hover on their name in the timeline and click on the drop-down menu.

Go down to "Plurk privacy & options" Click on "only" and private plurk away.

yeah, not very convenient (because it's easy to miss, unline new comments) but works. Can also add a comment for safety I guess

Looks like there is more than one way to Plurk a pvt message.

and, it's no worse than direct on Twitter, which is also way easy to plurk up.

move the cursor over your name and notice how the 'is' changes to a drop-down? click there and its an option.

woha, it took me a while to see that, but I reckon it works the same?

I think this feature is good, just like group chatting