I think you may try to make your desired cells read-only and editable accordingly, see the demo:
Also, you may try to protect a worksheet and then specify which cells you want should be editable, all other cells would be locked, see the sample code:
WebWorksheets sheets = GridWeb1.WebWorksheets;
WebWorksheet sheet = sheets;
//Since all the cell are locked by default same as MS Excel.
//Set A1 cell and B1 not locked.
sheet.Cells[0, 0].IsLocked = false;
//Set A2 Cell not locked
sheet.Cells[1, 0].IsLocked = false;
//Protect the sheet.
sheet.IsProtected = true;