> The pool would notice that certain participants contributions are conflicting with other discoveries, and ban such participants?

How? Or, you ban me, I sign up again under a different alias.

The pool can use a 2% fee for old accounts and a 20% fee for new accounts (for example, with less than 1 month or less than 10^x hashes calculated.)

Fees cost almost nothing to hostile miners. With a 20% fee, they get 100% of their hashrate through the theft, and 80% from the pool. So long as you pay anything at all to new miners doing this attack is beneficial.

But, you see, the attacker isn't mining anything of value under the pool, thus the pool has nothing to apply the fee over.

And so few new miners would sign up. Which would be good.