Manual review (depends on the scale you intend to go for), build a 'trust' profile of your users, analyze the crap out of sign up page activity to detect abusers even before they've made their first move.
None of those require extra user interaction and can be quite effective.
Do you have any suggestions for further reading about approaches to this? I do plan to do manual verification initially, but if the project is succesful, I won't be able to keep up- a good problem to have, and I don't want to optimize prematurely- but I also don't want the site to become overcome with spam before I have any clue how to handle it :)
None of those require extra user interaction and can be quite effective.