So you want a way to control spam, and you want it to have minimal impact. Ideally, the people who receive spam or have to process it wouldn't have any impact on what they do. I think that maybe the tax code can be helpful here.
Spammers do what they do because there's a very minimal cost to them. Upstream people pay the cost, of course.
When a business sends out direct mail, it doesn't just splatter that mail all over the known universe. It targets the mailing as precisely as it can, to reduce the costs of it. The company may choose to do a wide-ranging campaign, from time to time, but these are carefully managed so that the benefits (new business) outweigh the costs (of printing, mailing and so forth).
Let's say we put a flat-rate "send tax" on unsolicited business email of $0.05. That is, for each unsolicited commercial email, the sender must pay a tax of a nickel. You can see how that will add up. If a list is truly opt-in, then the cost will be worth it.
There are significant advantages to making this part of the tax code. The first thing you'll suppose is, well, the spammer isn't gonna count his spams! That will probably be true...but whoever is receiving them can find out where they came from, and a sending ISP can be forced to reveal the source of email pretty easily.
At that point there are only a few variables: Who sent the spam, and how many there are. Those are the only questions. Tax is due and payable, and there's no way out of it. I suppose you could permit a write off like any other expense.
The thing is, if a spammer doesn't report the spam, he's subject to an IRS ruling later on in his life, and that's something he probably doesn't want to do. It's one thing to be spending spam and pissing people off right and left. It's another thing entirely to be a tax cheat. Far fewer people are going to be willing to take the step of deceiving uncle sam.
Some of the tax dollars gained could be used to form a spam team, which collects spam and uses statistical methods to estimate the numbers produced by a given spammer. Example spams can also be compared against a spammer's list to see if they are exceeding what they said they'd send.
There are a lot of ways to trip up the spammer with this approach. The deterrent factor of the tax code is the real win -- there's plenty of law out there on that already!
So I say tax spam, at $0.05 per unit, and let the government do its thing!