I don't think introducing a rank would prevent them though. As Hilariousandco said they rake in millions and wouldn't care about paying a small one time fee in to enable them to post their announcements.
My suggestion is more about them financially contributing to the mess and workload they cause here. Why should I or other staff members be essentially running campaigns for them for free because that's what happens when they decide to pay anyone for anything and do nothing about spam and this can't and shouldn't be acceptable or tolerated. With that being said, these icos only take in the money here
after they've made their thread and at the moment it costs nothing to do that. If there's a substantial investment fee involved beforehand a lot will think twice and putting money upfront shows you at least they believe in it to some extent and not just a scam they've knocked up in five minutes because they've got nothing to lose.
The amount of money they receive through presales and 'donations' is insane and they would very likely be willing to pay a one time fee to be able to post. It could be a requirement anyway, just to cut a little percentage of the users who aren't willing to invest a little into their project.
If this is the case then they shouldn't have any issue in buying the membership. Most of them seem to be buying the Copper Rank already so I don't think them purchasing a higher rank to be able to create a thread in the ICO section would be an issue.