TC: I already said 2 ideas. Post either rating limit or ranking limit. It is possible and there is no limitation imposed by the site owner. You will have to make the calculations anyway before starting the tournament.

If you use the rating limit, you can include that to your tournament creation, so that you let the site handle the legitimacy of the participants. This convenience has been included to the tournament creation *exactly* to facilitate your kind of tournament and not just for show.