不客气 1) interj. could be the reply to "谢谢", as "You're welcome" & "Don't mention it" 2) adj. impolite, rude, blunt
别客气 1) is used while entertaining a guest. means "make yourself at home" , "help yourself","not at all" & "don't mention it" ,equals to "不要客气". 2) also it could be used to reply "谢谢",the same as "不客气".