Well, you obvious Google searched it, found something about it on some website who provided a registration link with its referral code. And normally, as Binance Exchange uses cookies as every website today does (at least to a certain extent) it memorized you and that's why you weren't able to get rid of it. It also prevents cheating, meaning being brought to the site with one referral and afterward wanting to assign with a different one.
Maybe you should read Binance Privacy Policy and take a look the paragraph about cookies.