safari blocked due to **** pop up demanding money

hi

my nephew was using my macbook and clicked on a link which took home to a **** website, my husband closed it immediately but I now have a pop up which has rendered him unable to use safari and can't get rid of this opo up, it is demanding a £100 to get rid of this thing blocking my safari, can anybody tell me how to delete it please. My nephew is only 8 he didn't know what he was clicking, my husband was very shocked when it bought up this **** site so I am just glad he was sitting next to him when he did it. I have a MacBook Pro but I am on my iPad in Spain and he is in England, if somebody could tell me how to get rid of it I can relay it to him to do. I had this happen to me quite a while ago but I can't remember how I did it, without being able to get to the computer I really can't remember how I did do it.

hope you can help

many thanks

Tracey