Step (1)

Suppose you have a bound field namely UNIQUEID that you don't want to be displayed then you just apply the following properties to your bound field.

<asp:BoundField DataField="UNIQUEID" HeaderText="Unique id" ItemStyle-CssClass="MakeHidden" HeaderStyle-CssClass="MakeHidden">
<HeaderStyle CssClass="MakeHidden"></HeaderStyle>

<ItemStyle CssClass="MakeHidden"></ItemStyle>
</asp:BoundField>

Step (2)

Then add the following css

.MakeHidden
{
display:none;
}