Employers should be able to freely communicate with candidates.
If I was looking for a job on a particular site and an employer wanted to communicate with me again (i.e., they are interested), I would be very annoyed if they couldn't contact me!
I understand that sending multiple communications to a candidate could be done in error, but it should still be allowed if the employer chooses.
What I would suggest is changing the error to a warning. Something like "You have already communicated with this candidate, most recently on <timestamp>. Are you sure you want to send them another message?"
In terms of rate-limiting, something like 1 message per month would be fine. (I gather these types of communications are supposed to be one-way to encourage a conversation via e-mail/phone/whatever.)
Another idea, which could be combined with the first, would be to have the same "You last contacted this candidate on <timestamp>", plus maybe "You can contact this candidate again in n days" appear in search results and/or on a candidate's page directly. (Note: I don't have access to the employer pages, so I'm guessing this isn't on there already.)
That way the employer could see when they contacted a candidate at a glance instead of attempting to send a message only to find out they have to wait a few more days.