You should be able to clear the DNS Forwarder cache by simply restarting the dnsmasq daemon as follows:
That will clear the internal cache, but your client PCs may still have cached entries. You can clear the DNS cache on a Windows PC like so, from Start > Run:
Other operating systems will surely have other means to clear the DNS resolver cache.
Browsers also have their own internal DNS caches separate from the OS. You may need to close and re-open your browser.
As a last resort, rebooting/restarting a client will surely clear any locally cached data.