First, I think you are making a mistake by planning to over-ride the expected behavior of the DataGridView when it gets an Enter/Return keypress while a cell is selected.
But here's a 'sketch' for code to use:
private char EnterChar = (char)13;
private void dataGridView1_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == EnterChar && dataGridView1.SelectedCells.Count == 1)
{
e.Handled = true;
}
}