Keeping in mind that the English language uses double letters often you probably don't want to blindly eliminate them. Here is a regex that will get rid of anything beyond a double.
Regex r = new Regex("(.)(?<=\1\1\1)", RegexOptions.IgnoreCase | RegexOptions.CultureInvariant | RegexOptions.Compiled);
var x = r.Replace("YOU SUCCCCCCCCCCCCCCCCCKKKKKKKKKKKKKKKKKK", String.Empty);
// x = "YOU SUCCKK"
var y = r.Replace("OMGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG!!!!!!!!!!!!!!!", String.Empty);
// y = "OMGG!!"
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…