Shoot those around you who talk?
On a serious note, the only way around this is to "fly high" but then if you're not allowed to open record, that's not gonna be a possibility. Omnis, by their nature, capture sound from ALL AROUND you where as the cards will pick up more of "what's ahead of you." Suffice to say, if you have some asshats three or four people in front of you, you're gonna hear them loud and clear on your capture with the cards and less so with the omnis...
There is no *EASY* way to get around asshats (except move and that's not always possible, either).
EDIT: Another thought... Doing a card/omni matrix from the same spot may yield worse results than if you stuck to one capture. Personally, when I'm outdoors, I go with the omnis. Stack taping can be good, however, depending on the soundfield (stereo or mono) at the source, you might end up missing some effects/panning if you're to one side.