.... less than 5 posts. This will hopefully encourage registration on the forum and for the user to start posting. For all passing surfers it will display a 728x90 ad block under the first post of each thread. Hpefully only a single ad block will ensure maximum paying ads to display. Take a look here : http://www.neforum2.co.uk/smf
I don't use simplemachines but after a quick look at there website i would say that the Ad Management mod looks like it may do what you want using permissions custom.simplemachines.org/mods/index.php?mod=255
I can't actually get that mod to install for technical reasons, so i've manually edited. I've actually changed it to <10 post limit.