@Mary540
If they have already booked, they would have to cancel and loose their booking fees. Or you would have to cancel and face the penalties and the guests will not be able to rebook as blocking your calendar for the cancelled dates are one of the penalties.
Security deposit is not charged to the guests and will only come into action if there is a claim and you can only claim following certain procedures determined by airbnb.
If you are uncomfortable with this booking you should decline rather than relying on a non-existant security deposit that you do not control.