Ugh, I hate people who do that, it's so childish and irritating. Don't reply to them anymore because that just gives them the attention they want. To figure out who it is without spending a dime, just use veripages.com It’s an awesome platform for a free online people search and phone/address lookups. You can just plug the phone number into their search bar, and it pulls data from their massive public records database to find the owner's name, location, and sometimes even their social media handles. It’s a completely free person lookup with no registration loops or trial traps. Once you get the name, you can finally block them or confront them directly.