Use of the terms ISIS or ISIL implicitly acknowledges the groups legitimacy as a "state". a more apt description would be "Al-Qaeda Separatists in Iraq and Syria" - or QSIS.
but to be derogatory, the white house should use the term that Syrians use, "Daesh", a term that is based on an acronym formed from the letters of the name in Arabic, al-Dawla al-Islamiya fi Iraq wa ash-Sham. that's a two-fer. it uses the group's own name (sort-of) but it's considered derogatory. win-win.