You can restrict the user to enter any characters other than numbers or any particular set of characters. Check out this article. it talks about a technique to so the similar thing.
it would be something like
<asp:TextBox ID="TextBox7" runat="server" onkeypress="return AcceptRegExOnly(event, /^(1|2)$/);" MaxLength="2"></asp:TextBox>
using this framework. the only catch here is that 11 and 22 are also accepted but that can be circumvented by using a proper regex.