You don't live in York, do you? There is a heck of a lot of heroin on the streets here, I've seen people getting the foil out in the city centre.
It's a difficult situation. If a cheap lighter, let them walk away, see if they come back.
Talk nice to them - there but for the grace of god, etc.
(and if you try to grab the lighter back, you might end up with them stabbing a HIV/Hepatitis-carrying needle into you)