onBeforeSplit

fires before merged cells are split

boolean onBeforeSplit(number row,number column,array value, [string page] );

row	number	the number of the row that the span starts with
column	number	the number of the column that the span starts with
value	array	the array with values of the deleted span
page	string	the name of the sheet

boolean

returning false will prevent splitting of merged cells

$$("ssheet").attachEvent("onBeforeSplit", function(row, column, value, page){
    // some code here
    return true;
});

Details

return false to block cells' splitting