Nothing needs to be done ... in the year of the excess contribution the excess should have been added to your wages on line 1 of the form 1040 so you could pay taxes on the excess withheld from the box 1 of the W-2. And then later, when you eventually take a distribution from the 401K you will pay taxes on the entire distribution ... you will get no credit for the taxes paid earlier on the excess contribution. Thus you are paying taxes on the same money twice and there is no way to mitigate this issue.
