Try this code to see if it gives you what you want.
private void Model_SelectionChanging(object sender, GridSelectionChangingEventArgs e)
{
if(e.Range.RangeType == GridRangeInfoType.Rows && e.Range.Height > 1)
{
e.Cancel = true;
}
else if(e.Range.RangeType == GridRangeInfoType.Cols && e.Range.Width > 1)
{
e.Cancel = true;
}
else if(e.Range.RangeType == GridRangeInfoType.Cells
&& (e.Range.Height > 1 || e.Range.Width > 1))
{
e.Cancel = true;
}
if(this.gridControl1.Selections.Count > 1)
this.gridControl1.Selections.Clear();
}