It goes like this; you are approached in a car park by someone claiming to be collecting for the deaf or some other worthy cause. The person will usually have a clipboard and fake identification. On the clipboard will be a list of names of people who have donated with the amount alongside. Not surprisingly, the handwriting will be the same for every person and the amount will be at least 10 euros.
When you refuse to donate, they become persistent and in some cases even aggressive. The advice is to stand your ground, get your mobile out and pretend to call the police at which point they will almost certainly disappear rapidly from view.
We have experienced the scam once in the car park at Eroski and at least twice with people at our gate. The first time we were new to the country and did hand over 5 euros. Since then though I have turned them away. We always keep our gate locked and only open it if we know who the person is or are convinced that they are genuine. It might seem rude to keep people at the gate but it is a lot easier than trying to remove them from the premises.
PS The Spanish word for beggar is mendigo or mendiga. You find them outside churches and cathedrals and in the streets. Mendigas also come and pester you when you are sitting outside cafes and restaurants. We have even seen them outside supermarkets. Sometimes they carry a baby or a young child is with them who they claim is hungry.