how to prevent from spammers while submitting web form in asp.net?

Question

i am working on a project developed in asp.net. In that one page holds the order details. Validation works well while form values filled up normally but some spammers are by pass that validations (All validations are done from client-side).

For example, i have one dropdown as "ProductName" and default value is "Please Select Product" and it is required field validator (validation on client-side only), but for some reason some spammers are by pass validation with that default selected value "Please Select Product".

It looks somthing like this:

ProductId: 1
ProductName: Please Select Product <== here it must be a product name
Address: As Specified Above

How to resolve this problem ?

Thanks

Answer 1

First You should have CAPTCHA implemented on every input form page available to Un-Authenticated/Public Users.

secondly, you should validate your inputs on server side as well. in ASP.Net with Page.IsValid and in MVC.Net ModelState.IsValid