Removing characters from end of string from character to character
You could simply take advantage of the string.LastIndexOf() method together with the string.Substring() method like so  

private static string Clean(string value)
{
    return value.EndsWith(")") ? value.Substring(0, value.LastIndexOf('(')).TrimEnd() : value;
}


and use it like so  

foreach(var i in methodResponse.Response.Data.Results.Where(x =&gt; x.CategoryFriendlyName == platform))
{
    i.BoxName = Clean(i.BoxName);
}  




Some side notes about the posted code:  


Don't use abbreviations when naming stuff e.g var arr  
You shouldn't check like while(exit != true) but rather like so while(!exit)