You can either use an asp validator or limit the user to type on your date textbox.
Using validator:
<asp:CompareValidator id="dateValidator" runat="server"
Type="Date" Operator="DataTypeCheck" ControlToValidate="dateTextbox"
ErrorMessage="Please enter a valid date.">
</asp:CompareValidator>
Using user input limit :
Since you're using Ajax calendar, check your TargetControlID property and set it to readonly.
Good luck!