Warning BT user your computer is at risk, ring Apple on 0800 1120057

Hi all,

I was surfing for a new boiler today and a screen opened with a stark warning saying that as a BT user my computer was insecure and my bank details etc were vulnerable. another little pop up opened telling me to contact Apple on 0800 112 0057 to resolve the problem. The address in the address bar read 162.243.40.201. I realise that this is a scam but the troubling thing was that I couldn't close the pop ups. In the end I had to force quit Safari and start again.

Is there any way to close these down when they open (without for closing Safari) or to stop them in the first place?

Thanks.