Owww... that's a bad software design. It should not be possible to get into queue without paying. What directory software are you using?
Weird weird weird... I did not think that worked on phpLD.
I would definitely post that as an issue on the phpLD forum.
Wait... phpLD 2.x or phpLD 3.x?
Frankly speaking, I have a free directory, Hira SEO Directory and I can say, that spam will always be a problem when it comes to free directory.
I think if you gain traffic, say 1000 unique visitors daily, then you can sign up with a CPM advertiser and earn revenue from your traffic.