Actually Varnish is great here, one normalises the requests and retains only the querystrings that are valid for your application filtering out (removing) all those that are not valid.

The key thing is, you know your application, and you know what the valid keys are and the valid value ranges. If you can express that in your HTTP server and discard requests then it can be very cheaply done.

A forum really doesn't have that many distinct URLs, and so this is easily done. It would be harder on a much more complex application, but the original question related to these smaller side-project applications.