dnsmasq keeps track of the DHCP leases it has in a file which is defined by the dhcp-leasefile config option.
To delete a lease from dnsmasq, first stop dnsmasq.
$ sudo systemctl stop dnsmasq
Remove the lease from the file. On Ubuntu this lease file defaults to /var/lib/misc/dnsmasq.leases.
Once the lease is removed, start dnsmasq.
$ sudo systemctl start dnsmasq