Is there any way you can refine the registration process? Can you make it so you can ask some questions of the member trying to sign up, based on what they answer can determine whether or not you approve the membership.
This is already implemented. People trying to register have to do a simple piece of arithmetic which changes randomly.
The users who are putting up the inappropriate advertisements are not robots but have actually had to manually register. Sadly such people parasitise discussion forums and if not immediately dealt end up overwhelming the serious discussions. These users seem to emanate from China.