Just post a message that says "thank you" or something.
Voting ought to be anonymous, for lots of reasons, most of
which have been hashed out in the real world.
(I mean, if it weren't anonymous, nobody would EVER
give merlyn or vroom a -- for fear of retribution)
No seriously, if you want somebody to know that you thought
they had a really great post and you voted it up because it
was so well-reasoned or whatever, it's trivially easy to
just /msg them. I've done it before, it works well.