@Kristina372 If you are the holder of the Host account associated with the bookings, you can remove your agent's access as a co-host without affecting your existing bookings.
But if the agent was using his own Airbnb account to manage your property, you have a more complex problem that will require the intervention of Airbnb's customer support (this forum is only visited by peers, not Airbnb employees). I don't think there's a way to transfer a booking from one host account to another, so these reservations might have to be cancelled.