WARNING WARNING WARNING PRIZEGIVEAWAY LOCKS SAFARI

I found, using my PC, that others have been struggling with this.
I eventually got rid of it by force quitting Safari disconnecting from my router. Rebooting and going to Safari Preferences and selecting Remove All Website Data.
I don't know if this was the correct thing to do but it seems to have worked.
There is some advice on the web saying go to System Preferences > Flash Player and block locally stored objects but I cannot find FlashPLayer in my Preferences and don't recall ever having installed it.
Has anyone else come across this?
Does anyone know or can surmise if it is likely to have done any invisible nasties?