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