Hi Neda,
If you want the User cant enter then
<div>
<Ajax:ToolkitScriptManager runat="server"></Ajax:ToolkitScriptManager>
<asp:TextBox ID="txtRestrictSPChar" runat="server" />
<Ajax:FilteredTextBoxExtender TargetControlID="txtRestrictSPChar" runat="server" FilterType="LowercaseLetters,UppercaseLetters,Numbers" >
</Ajax:FilteredTextBoxExtender>
</div>
This is the best and simple solution as well,
Thanks and Regards,
Rk_Hirpara