System.Uri is your friend here. This has many helpful utilities on it, but the one you want is GetLeftPart:
string url = "http://www.website.com/default.aspx?utm_source=twitter&utm_medium=social-media";
Uri uri = new Uri(url);
Console.WriteLine(uri.GetLeftPart(UriPartial.Path));
This gives the output: http://www.website.com/default.aspx
[The Uri class does require the protocol, http://, to be specified]
GetLeftPart basicallys says “get the left part of the uri up to and including the part I specify”. This can be Scheme (just the http:// bit), Authority (the www.website.com part), Path (the /default.aspx) or Query (the querystring).
Assuming you are on an aspx web page, you can then use Response.Redirect(newUrl) to redirect the caller.