onBeforeSpan

fires before cells are merged

boolean onBeforeSpan(number row,number column,array size, [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
size	array	the number of cells in col and row spans
page	string	the name of the sheet

boolean

returning false will prevent cells merging

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

Details

Return false to block cell merging.