The site can state if it deems something is not acceptable.
Where shall we start?
Weapons of any sort
Catapults
Fur Coats, and maybe leather shoes too
Sexual materials
Lazer pens
Animal repallants
Animal traps
Any product tested on animals
Fishing equipment
Beer cans (4 packs)
Anything that could be dropped as litter
Dogs
4X4's
All adverts appear because of the topic of the page, hence if a page is dedicated to insects, you will probably get an advert for insect repellant.
Hence if you look at Coley's excellent thread on 4X4's you will see adverts for 4X4's
Each time somone clicks on the ad the site gets a small payment for providing the link, so anyone who has clicked on the link has contributed to the site and probably enticed the advertiser to keep trying to advertise on here, so it probably was not wise to provide the link in the thread???