My solution isn't perfect, no solution is, but I pretty much am following the KISS line to get here.

You need something that puts the burden on the site admins and a group of volunteers rather than something that puts a burden on the site's visitors.  It is the visitors, after all, who generate the revenue.  The easier and more friendly your site is to navigate and use, the more visitors who will come and the more who will come back.

Throw in a ton of bot verification and download security steps and you'll lose a lot of interest, which also means you lose a lot of revenue.  So, I think the proper place to put the burden for increase security is on the admin side, not on the user side. 

Now, no automated approach on the admin side is going to stop the scammers.  Unless you place the burden on the users this isn't going to happen.  You'll need to have things like bot checks and or only registered users downloading to prevent scams if the security system is to be automated, which places the burden for the additional security on the users. 

If you want to place that burden on the admin side, your going to need people to police the site.  Rating systems are great but again you run into multiple problems relying on a rating system, most of which have already been listed, but bottom line on a rating system is that it can be manipulated by the unscrupulous.

So your back to square one, which is having people police the site.  This leaves you really with two options, either verify everthing that is uploaded (such as renderosity does now) or go to a multi-tiered system.

Verfying all uploads is tough - first theres the time commitment and then theres the judgement call made on each and every item.  But eventually no matter what such a system will bottleneck your uploads and again choke off your revenue. 

No matter what sharecg decides to do I for one will remain a part of the community.  But I think any system making it more difficult for people to download content is a bad direction to go in, IMHO.  As someone else mentioned, storage space on the internet is getting less and less expensive every day.

My choice of sharecg as a hosting site was based primarily on the fact that you didn't have to sign up for an account in order to download anything.  I didn't want people who were coming here from other sites directed by me to be hassled with filling out a lot of forms.  As always, just my 2 cents worth.