Yes it is bannable, I got banned by having 100 refs (not all botted I had some friends do some for me too, some of them were 30 lvl or less). I bought refs from different people. It is risky. And you may get banned. No services like this offer a refund in case of a ban, so it is on you if you want to risk or not. I wouldn't do it if I were you because I had 100 refs for like 6 months, and when they announced that they were releasing a new RAF system, after a few days I got banned. They took some of my skins and champions, they reduced my RP to 0, and took away some of my IP as well, and they took the rune page I got from the RAF system too. But it's your choice if you want to buy or not, good luck.
But do not ask questions here, it is not allowed. There is a speical forum for that which you can find here: