As much as I'd like to side with the guy getting picked on (Freshdesk), Zendesk is completely right. Ignoring the interface and naming similarities, you can't complain about ZenDesk on Twitter without multiple Freshdesk spammers bombarding you. Sure there's nothing illegal about it, but they're obnoxious with the "a customer of a rival is complaining, I MUST CONVERT THEM!" concept.
(I called them a rip-off of Zendesk back in late October - http://mobile.twitter.com/ecaron/status/123594375560302592 - and it took several days for @vshankar90 and @mrgirish to stop harassing me on Twitter...)