How do you tell ex wife not to sit with us at kids games?
If you have sons in sports then simply be kind and tell her that you would like some ‘guy on guy quality time’ and could she just let you and your son(s) go together. Perhaps ask her to meet you and your son(s) for something to eat later after the games. If you have a daughter or two then she has every right to be there and to ask her not to be would be a slap in the face to her.