Remove a value from querystring in MVC3 and redirect to the resulting URL

I think you approached the problem correctly.

This is the code that worked for me:

public void OnAuthorization(AuthorizationContext filterContext)
{
    if(filterContext.HttpContext.Request.QueryString["some_key"] != null) {
        RedirectToCleanUrl(filterContext);
    }
}

private void RedirectToCleanUrl(AuthorizationContext filterContext)
{
    var alteredQueryStringPairs = filterContext.HttpContext.Request.QueryString
        .Cast<KeyValuePair<string, object>>()
        .Where(pair => pair.Key != "some_key");

    var routeValueDictionary = new RouteValueDictionary(
        filterContext.RouteData.Values.Union(alteredQueryStringPairs));

    filterContext.Result = new RedirectToRouteResult(routeValueDictionary);
}

I used the answers from these questions to clean up your initial code:

how do i access query string params in asp.net mvc?

Make NameValueCollection accessible to LINQ Query

Here's the code I ended up writing:

protected void StripQueryStringAndRedirect(System.Web.HttpContextBase httpContext, string[] keysToRemove)
{
    var queryString = new NameValueCollection(httpContext.Request.QueryString);

    foreach (var key in keysToRemove)
    {
        queryString.Remove(key);
    }

    var newQueryString = "";

    for (var i = 0; i < queryString.Count; i++)
    {
        if (i > 0) newQueryString += "&";
        newQueryString += queryString.GetKey(i) + "=" + queryString[i];
    }

    var newPath = httpContext.Request.Path + (!String.IsNullOrEmpty(newQueryString) ? "?" + newQueryString : String.Empty);

    if (httpContext.Request.Url.PathAndQuery != newPath)
    {
        httpContext.Response.Redirect(newPath, true);
    }
}

You might also want to UrlEncode the query string params, but I'll leave this up to you.