if someone behaves 'familiar' with you when you don't know them

i've had this sometimes in life online and in real life, someone who behaves in a familiar way with me when i don't know them and they do not know me, i find this really irritating and annoying....how should you react to a situation like that?

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Tumbleweed · F

On here, it annoys me at first but I'll go along with it til I decide Nope. But I've made some really good friends in a situation like that.
In real life, I find it sort of creepy so I ease away as best I can.