If you have a list record and have many user use it for data entry.
You don't want user to merg cell in that Sheet.
This is the solution :
1.Save your excel as Macro enable file type (*.xlsm).
2.Go to Developer tap and click VB code.
3.Find the sheet you want to disable merg cell.
4.Then copy the Code below to that Sheet (vb code)
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Count > 1 Then Target.Cells(1).Select
End Sub
5.Save and Test.
Other method is
1.Please go to review tab in excel and Share workbook .
So your excel will gray out of Merge excel icon in the work sheet.