Presumably you would still be able to spoil your ballot, or does the bizarre American obsession with stupid voting machines get in the way of that?
The Diebold machines that Georgia uses will allow you to leave a race blank, but don't let you spoil your ballot in any other way. The older machines that Louisiana used (it was a giant paper ballot, about four feet wide, mounted on a machine where you click a candidate's box and a little red X lights up) had another little red X in the top-left corner of the ballot item that would light up if you didn't have the correct amount of boxes checked for that race (I.E., 2 for the New Orleans at large city council seat(s) and 1 for everything else) but it let me submit an undervote so I'm pretty sure it would let you submit an overvote as well. Louisiana doesn't do write-ins, though, but that's not as big of a deal because it's incredibly easy to get on the ballot in the state.
