onBeforeSpan

fires before cells are merged

void onBeforeSpan(number row,number column,array size);

row	number	the number of the row that the span starts with
column	number	the number of the column that the span starts with
size	array	the number of cells in col and row spans

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

Details

Return false to block cell merging.