I'm not going to reply back to a former friend
Why don't you want to reply to this person?
I'm just tired of getting messages all the time.
Having to frequently reply to messages can be annoying but you shouldn't ignore people. Your friend could really feel lonely right now and wouldn't it be kind to give them some attention?
You're too nice, but I don't wanna do it.
Your former friend could get worried about you, and start trying to contact you in other ways. A lot of people would say the best way to handle something like this would be to let them know you're okay and that you are very busy right now and can't talk, then they'll most likely leave you alone.