Unfortunately, one of the most notorious signature campaigns, (the one I am currently involved in at the moment,) has the help of a Global Moderator, and it is still heavily abused. Poor guy has to keep playing wack-a-mole and have hundreds of accounts banned from the campaign, and the spammers still crop up like so many weeds.
It's why campaigns need to set the entry barrier high as well. Don't accept low ranking members and check the post quality of those applying so that you don't have to keep kicking people out.
Only in Bitcoinland do people complain about a 20 month ROI. Rest assured that signature campaigns would have to pay more.
I'm fairly inexperienced when it comes to ROI'ing on things, so I don't have anything to compare that to. It just seems like a lot of time and a lot of posts.