I did something like this:
var range = $("#spreadsheet").data("kendoSpreadsheet").activeSheet().range("A1:A100"); //Get the range of cells I want to remove validation from.
range.validation(null); //Null out the validation
The only problem with this method, is that you need to re-create the spreadsheet widget after making a change like this. (it may be fixed in a more recent build)
var storedSheet=$("#spreadsheet").data("kendoSpreadsheet").toJSON(); //Keep the sheet in JSON format for rebuilding later.
$("#spreadsheet").data("kendoSpreadsheet").destroy(); // destroy the Grid
$("#spreadsheet").remove(); // remove all Grid HTML - might be unnecessary
Then use the json spreadsheet data that you stored in storedSheet to rebuild your spreadsheet widget.
I don't know if this is still the most effective way to do this, but that's what I'm using.