Clause 3: To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes;
You notice that it does not say with in the states.
Do you have any evidence to support your position?
Legalizing only marijuana will not eliminate the need for the DEA.
Who said it would? Returning the rights to the states would elimate the DEA.
It would be better if you read this case yourself:
http://www.druglibrary.org/olsen/DPF/visman.html
Seriously. It's not long, and should help you understand.