How smart are these offshore spammers though? Presumably smart people would find better work?

Based on what I see on my site they are pretty smart and very determined.

I think you should make it truly merit based - submit a bugfix or a feature patch to an open source project to post a link :)

You can vary the task based on the sort of news site you would like to have. To compete with programming.reddit.com, you could use merit-based news with your suggestion.

As for puzzle solving, you can make the puzzles really difficult to eliminate the vast majority of spammers.