How to stop malicious Javascript from Executing?

Hello,
I have a web application where I want to stop any malicious JS to execute.
For example:
http://www.mywebsite.com/default.aspx?ID=<script>alert("hello")</script>
If a client does the above then an alert box pops up on the client screen. How to stop that.?
Also I have set validateRequest="true" in the machine.config but still the JS does get execute.

Thanks.

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