Actually no. You can use a challenge/response technique. Take the MD5 hash of the current time plus some secret "seed", and send that to the client. Then, whenever the client requests that action, it must supply this MD5 hash. On the server side, you recalculate the hash for each of the last x seconds, and see if any matches what the client supplied. Only permit the action if the hash matches.

The trick is to only deliver a hash to the client when it is a valid time to perform the action. And by restricting x to a small number of seconds, you control fairly tightly when the action can take place. So even if they do fake it, they can only fake it when it's a valid time to perform the action anyway.