You don't have to reject their offer, you have to ignore most of these offers that comes to your messages. Who on Earth is going to give you 12 Bitcoins? That is $1.1M in valuation of the current price without anything to ask for? or let's say you pay them $1k and then what's next? they run with your money and that 12 BTC is just a dream that will never come true because there is no 12 BTC that will be given away that easy. Sometimes, we need to analyze if things are being taken for granted or people are trying to fool us.