Your mistake was not ignoring this when it first started. The fact that you got upset and made it known to them, means they're going to keep doing it. They get off in annoying people.

I'm surprised that after this long, people STILL don't know how to deal with trolls. In case you don't know, the ONLY EFFECTIVE WAY is to not react. Literally do nothing, carry on and pretend they aren't there. This is 100% guaranteed to make them give up and move on. But it's too late for that now. You're just gonna have to deal with it.