@sj093, there is no way to manually delete a lease in the MX DHCP server. The lease will be removed, and thus the IP address made available for another system, when it expires.
As others have said, your two options are to use shorter lease times, or a bigger subnet. If the systems attaching to this network are changing regularly then you want to shorten your lease time. If it’s that you have a large number of clients connected then you could move to a /23 subnet, or consider whether you should be introducing another VLAN/subnet.