I have a working solution, that uses deleteRows(). I'm not sure why that didn't work for you, it would be interesting to see your code.
The new Forms product (February 2013) is quite different than the legacy forms - this solution will work on legacy forms only.
Anyway, here's what I have, with before & after screenshots. The tidy() function can be called by passing in the number of rows to be removed, or you can call it from a menu as demonstrated here. A more complete version of this script is available as a gist.
/**
* Standard onOpen provided in script template.
*/
function onOpen() {
var sheet = SpreadsheetApp.getActiveSpreadsheet();
var entries = [{name : "Read Data",functionName : "readRows"},
{name : "Tidy Form Responses", functionName : "tidy"}];
sheet.addMenu("Script Center Menu", entries);
};
/**
* Prompt user for the number of form response rows to remove, then delete them.
* Assumes that the form responses are in the active sheet, that there is one
* row of 'headers' followed by responses (in row 2).
*
* @param {number} thisMany (Optional) The number of rows to remove. If not
* specified, user will be prompted for input.
*/
function tidy(thisMany) {
if (tidy.arguments.length == 0) {
thisMany = Browser.inputBox("How many responses should be tidied?");
}
var sheet = SpreadsheetApp.getActiveSheet();
sheet.deleteRows(2, thisMany);
}
Before

During

After
