how do I get rid off an annoying person
Online? Ignore them
In real life? Ignore them
It's really not as difficult as it seems. If a person feels tehy are not wanted, they do eventually go away.
You might want to consider something else, though: Why is this person annoying you? Are they desperate for friends? Reaching out for someone? It could be that they need your help and you're just leaving them to despair. There is always more than meets the eye.
ignore them, if they still stick to you like super glue, then tell them that they're annoying and that you don't want them in your life any more (easy)