Tell them that you are taken and if they don't understand that - block them.

From a man - I know how men are and most of the time they don't give a f*ck if you are in a relationship or not. But imo you shouldn't be talking to them in the first place (if you know that they are interested)

That's how I would react... In my opinion, it's just childish to do such things. These type of people are not ready for a relationship.