The fees actually paid (by people who are presumably not spammers) may be a bit high, though: I feel like it should be about a tenth of that.
In that case: why don't you lower it further?
It doesn't feel right though to clear all Evil units for a low fee (as that would benefit spammers). Hence my suggestion to allow only 1 registration after paying a smaller fee.Can you have a way to track how many people registered, were prompted to pay a fee, didnt pay, and subsequently registered a different account (both that did and did not require a fee). Something other than IP address would need to be used to track this.
That would go against many things this forum stands for.
Further reading:
A year or two ago I was researching fingerprinting techniques that'd work against pretty much anyone with JavaScript enabled, and I found several promising leads on that front. But then it occurred to me that I don't really want bitcointalk.org to be known as the #1 forum on the leading edge of de-anonymization technology, so I stopped pursuing it seriously...