Stop responding to the flirtation or games. Treat this friend as platonic as possible so that they understand that their games aren’t working anymore or you simply see them as a friend and nothing more.
This will make the person feel as though there is no point in playing games or flirting with you and they will stop as a result.
As long as you are consistent with this behavior, you will eventually come to realize that you aren’t loving this person the way that you used to because you are no longer at the receiving end of all the teasing that they had been doing previously.
All that teasing with the games and flirtation used to give you hope for a romance with this person and now that all of that is missing, it will be easier for your mind to finally be at rest and accept that this person is nothing more than a platonic friend.