Every plan I have so far seen to tax spam was basically to tax ALL email by a very small amount (1/10 cent) and those that send up huge amounts of it would get hit hard by it. You might want to take note that a lot of spam gets sent from the computers of victims whose computers have had a program implanted on them. They might use their computer without ever knowing it is there until they get a huge tax bill from Uncle Sam.
And since the tax system in the US is a guilty until you prove yourself innocent, this suggestion will penalize the victims.
In the words of another American, "Government isn't the solution to the problem. Government is the problem."
Do you honestly think the government needs yet another tax revenue stream? Don't you think the goverment already takes enough of our money?