I'm a member of an online community/forum. What that forum does to combat spam is simple in my opinion. Before any new member can gain full access to most part of the forum they have to post four times throughout the forum. The posts have to be relevant and they are allowed to post in one sub-forum per day.
For instance, on Day 1, I post in the Introduction section and introduce myself.
Day 2, I post a recipe in the Breakfast Recipes.
Day 3, I post in the Health Research.
Day 4, I post in the Movie Spoilers.
But what I noticed is that forum is more heavily patrolled by admins, who aren't afraid to use the banhammer on people who are breaking the rules or spamming.
Maybe more admins will circumvent this problem.
Just my two cents.