The OSD "Today's Meetings" and Touch 10 "Meetings" icon usually appears when TMS is the provisioning mode, there isn't a way to turn either off that I'm aware of, even checked the API guides just now.
UPDATE: The command you're looking for is below, however it's possible that both the OSD and Touch 10 will show the meeting info again whenever it polls TMS for a list of bookings it's scheduled for.
xCommand Bookings Clear
SX80 CE8.2 API Guide