example:
input:<NPS><UNKNOWN/></NPS> <NPS>test</NPS> <UNKNOWN/> <NPS><UNKNOWN/></NPS> test
output:<NPS>test</NPS> <UNKNOWN/> test
input:<NPS><SPN/> <UNKNOWN/></NPS> open Facebook <NON/>
output:open Facebook <NON/>
input <NPS><UNKNOWN/> test <UNKNOWN/></NPS> <NPS>test</NPS> <UNKNOWN/> <NPS><UNKNOWN/></NPS> test
output:<NPS>test</NPS> <UNKNOWN/> test

foreach (string input in new[] { "<NPS><UNKNOWN/></NPS> <NPS>test</NPS> <UNKNOWN/> <NPS><UNKNOWN/></NPS> test",
                                 "<NPS><SPN/> <UNKNOWN/></NPS> open Facebook <NON/>",
                                 "<NPS><UNKNOWN/> test <UNKNOWN/></NPS> <NPS>test</NPS> <UNKNOWN/> <NPS><UNKNOWN/></NPS> test" })
{
    Console.WriteLine("input: {0}", input);
    string output = Regex.Replace(input, "<NPS>.*?</NPS>", m => m.Value.Contains("<UNKNOWN/>") ? string.Empty : m.Value);
    Console.WriteLine("output: {0}", output);
}