I've seldom had someone refuse to leave THIS bluntly, so I had to think about this for a moment!! Not fun if they're causing trouble...
Anyone being ugly to me gets security called today.
If they're insistent and pleasant, sometimes I get a bit graphic in explaining what I will be doing (ie starting the IV or putting the foley or NG in) and hope the uncomfortableness communicates to the rude family.
If that isn't working I smile sweetly and say "You know, I always perform better WITHOUT an audience, but if ______needs you here I'll manage". Many times the patient then tells them to go.
If they don't...well....we're SOL and do the best we can. Luckily MOST folks are interested and truly want to be helpful...it's tough on the floors with 8-10 patients where you simply don't have time to give to these family members , but in ICU we have 2 patients and have more time for family support and teaching (usually). Sometimes family will even help: hold hands, etc. I've also had them change their minds quickly and run out...especially with ETT suctioning.....
With long term, chronic patients it may be helpful for all to include family in some care... as they begin to see the true situation and begin to understand...ie post CVA who will not get better, etc.
I like policies that support nurses (including set visiting times) but as everyone has mentioned, this is not what we get anymore....visitors even want to stay for codes today, some bring tape recorders and video cameras. <sigh>