You can either call or email again and say, "Sorry to put the pressure on, but I have to have a final count by tomorrow (or Wed, or whenever you decide). If we don't hear back from you by then, we'll have to count you as a no. Please feel free to email or call at xxx-xxx-xxxx."
Give them a deadline, and count them as a decline if you don't hear back. If they DO show up, you can either turn them away at the door, or let them attend but explain that you don't have a bag for them because they didn't let you know they were coming.